The Specificity of Vesicle Trafficking: Coat Proteins and SNAREs

Proteins that are destined for the secretory system usually begin their journey at the endoplasmic reticulum (ER), where proteins are translocated across the ER membrane into the lumen, before being selectively removed from the ER and packaged as cargo into transport vesicles bound for the stacks of the Golgi complex. Protein cargo then passes through the Golgi to the trans-Golgi network (TGN), where proteins destined for the vacuole (or, in the case of mammalian cells, the lysosome) are sorted away from cargo intended for secretion or for localization at the plasma membrane. This journey, from the ER to either the cell surface or the vacuole, is referred to as the anterograde pathway. Because protein transport through the endomembrane system is not a one-way trip, however, some vesicles also carry proteins in the reverse direction. This retrograde pathway is essential for the recovery of proteins that may have escaped from other endomembrane compartments and also for the recycling of the machinery involved in anterograde transport (see Battey et al., 1999, in this issue). Other endomembrane compartments are found as intermediate locations between the well-known organelles. For example, cargo destined for the vacuole may first pass through a compartment between the TGN and the vacuole called the prevacuolar compartment (PVC). The PVC is sometimes called the “late endosome” (especially in mammalian cells) or just “endosome” (especially in yeast). Throughout this review, we use the term PVC for this compartment, reserving the term endosome for the initial sorting compartment of the endocytic pathway. The first target for vesicles endocytosed from the cell surface is the endosome. Material within the endosome undergoes sorting such that cargo destined to return to the plasma membrane or continue toward the TGN is separated from cargo that will be degraded in the vacuole. The cargo intended for degradation first passes through the PVC, where it mixes with anterograde cargo that is also in transit to the vacuole. In this way, a single compartment can carry simultaneously both anterograde and retrograde cargo, indicating the intricate connections of the secretory system. A diagram of the compartments of the endomembrane system is shown in Figure 1. With traffic moving in both directions between organelles, how a particular cargo is packaged into the correct transport vesicle and how this vesicle is able to differentiate the correct target membrane from among all the others are important questions. Selection and packaging of cargo at the donor organelle depend on various coat proteins that assemble onto the donor membrane surface and mechanically form the transport vesicle. However, it is not the coat proteins that determine the target of a transport vesicle; instead, this is the role of proteins called SNAREs, which separately reside on the vesicle and target membranes. Together, the coat proteins and the SNAREs coordinate the trafficking of the cargo between the various organelles of the endomembrane system. Transport vesicles are formed at donor organelles through the action of several distinct coat proteins. Each type of coat protein is made of specific “coatomers,” and each is somewhat specific for a particular donor organelle, although some coats function at many different donor membranes. For example, a single type of coat, made from the COPII coatomer, is involved in forming vesicles at the ER, whereas a second coat, made from the COPI coatomer, forms vesicles from the Golgi complex (reviewed in Schekman and Orci, 1996). In contrast, many different types of coated vesicles are formed at the TGN, each with a distinct class of cargo destined for the vacuole, the PVC, or the cell surface. Retrograde transport, either from the cell surface, the endosome, or the PVC, also relies on coated vesicles. Coatomer components generally are recruited to the surface of the donor membrane through the action of a small GTPase that, upon binding GTP, associates with membranes and directs the assembly of the coat. Coatomer assembly mechanically drives the formation of a membrane bud, which subsequently pinches off from the membrane to form the transport vesicle. Cargo, which concentrates in the region of the membrane bud in a poorly understood manner, becomes incorporated into the lumen of the transport vesicle. Coated vesicles are not competent for fusion with the target compartments, so the coat must be depolymerized before fusion with the target membrane. This disassembly is believed to be stimulated by hydrolysis of GTP by the membrane-associated GTPase. The disassembly of the coat probably exposes the targeting machinery on the vesicle surface (i.e., the SNAREs) for subsequent delivery of the cargo to the appropriate organelle. The Endomembrane System of Yeast. The endomembrane system consists of seven major compartments: the endoplasmic reticulum (ER), the Golgi complex, the trans-Golgi network (TGN), the vacuole (or lysosome in mammals), the plasma membrane (PM), the prevacuolar compartment (PVC), and the endosome. In yeast, each of these compartments contains one or more which function to cargo intended for that The term is to distinct of membrane proteins that type of the is found on vesicle membranes. of the second the are found on the target membrane. and are into their membranes by either a or by of types of SNAREs that between and In of particular and through their to the that is a of vesicle Each of the organelles in the endomembrane system contains a particular of SNAREs that in specific to coordinate transport among the The are of the donor also are found in transport vesicles and on the target membrane. the other the are more and can be for the target membrane in which For example, the of all of the have (reviewed in Each of the major endomembrane organelles of found to one or more is to have the of proteins for traffic through the endomembrane system. (i.e., of some of the that in mammalian and in are shown in Figure and to have a of SNAREs or have forms of the single found in and of these SNAREs may be or their may the more endomembrane system of and The SNAREs many other are to their that the cell to SNAREs are the and the for and is from one of the of these proteins as membrane for a to be to all in yeast, and et al., et al., et al., In to these other proteins are to SNAREs in targeting a membrane protein from the and a small GTPase of the (reviewed in and The for how to membrane fusion and and et al., is in Figure Transport vesicles carry and a form of whereas the target membrane contains a bound to The the the for with the The and then through their that may to fusion of the The then forms a binding for which to the complex. the of the complex, the for recycling to the donor compartment and the for subsequent fusion The secretory which the ER and the Golgi complex, is the of assembly for proteins. a of (see and 1999, in this protein cargo is and as it passes through the ER and the stacks of the Golgi complex. For example, in the ER lumen, many proteins that can formation and the of a to specific In the Golgi complex, this may be or removed by various and in the various of some proteins also can in the stacks of the we on the role of the ER and Golgi in the of and of the the of and localization as as and of have in mammalian and The made with the in the by from of generally have to the compartments as in that in the may be to either the vacuole or the PVC, on cell type by of that have in the are with proteins destined for the secretory system first at the in the ER, proteins that are intended for the Golgi complex are believed to be sorted into distinct of the ER, where and 1996). The by which this cargo is packaged into COPII vesicles is in by and in this In the COPII are made of protein (reviewed in Schekman and Orci, 1996). The is recruited to the membrane through the action of a small which associates with membranes in a of COPII vesicles can be in with and the et al., is that is stimulated by of the GTPase of through the action of The form of is not able to with which subsequently to of the from the vesicle. components of the COPII components and the have found in and et al., The of trafficking in is by and which are believed to function in anterograde transport to the et al., et al., Each of these can form a complex with the fusion through the delivery of cargo to the (see Figure of the involved in this have from many et al., as a of the from and Because of these components also are found in mammalian et al., it is that the of transport is among the The of The and GTPase with the membrane the and on the surface of the target membrane. to this the is and specific between the and vesicle and in to the are in the Endomembrane The endomembrane system consists of the ER and the stacks of the Golgi complex. are the components in of the SNAREs are in transport from the ER on the COPII to vesicle In cells, these vesicles carry the and each of which can complex with the appropriate of the transport through the Golgi to by of the stacks in although this may not be the case in all (see Retrograde transport from the Golgi to the ER on the COPI to form vesicles that carry the with the of the ER the Golgi COPI vesicles carry one of or Each of these with the The Golgi is a complex of membranes that to in a as protein cargo from the to the trans-Golgi The of stacks in the Golgi complex can from to cell to and within the upon and other and this the secretory protein cargo to through the Golgi complex from the to intermediate to various as it the in the retrograde pathway is moving in the so that it is in specific Golgi stacks or is to the the cargo through the Golgi stacks the various stacks distinct of in of a in which the stacks into stacks vesicles the by many et al., Other of transport et al., or connections et al., between stacks as that anterograde in and may to some on this of the Golgi membranes in these is that are the of formation for a second type of coated vesicle that a COPI a coat (reviewed in Schekman and Orci, 1996). The COPI coat first in mammalian also found in The COPI is made seven and the small GTPase to COPII coat the form of associates with to of and subsequent of the coated vesicle. A of the COPI components also have in et al., COPI vesicles are for retrograde transport from the Golgi to the ER, a essential for recycling of the anterograde trafficking machinery and for of proteins (reviewed in and recycling is in by the et al., of which also are found in and and et al., In cells, COPI vesicles carry the also a role in the anterograde which with the et al., Figure of the ER have found in mammalian however, shown that and to be on the mammalian ER et al., vesicles have in (see et al., the of the trafficking machinery have not The role of COPI vesicles in anterograde trafficking within the Golgi complex is COPI vesicles have to both anterograde and retrograde traffic between the stacks of the mammalian Golgi complex and Orci, 1996). distinct of COPI vesicles that carry either cargo moving in anterograde or cargo moving in a retrograde have found et al., Because a role in the of cargo, how the can distinct of cargo and the vesicles for trafficking in different directions is not although it is that transport a that on the intermediate Golgi stacks found in it is that all the have in mammalian and cells, the of all of the Each of these and to to intermediate Golgi stacks (reviewed in that cargo transport from the to stacks of the Golgi complex a with COPI vesicles being for retrograde trafficking of have that some cargo can be to form at the Golgi stacks and be through the Golgi stacks as a et al., from the TGN to the Protein the of anterograde transport to these somewhat from and of that both are in mammalian cells, or that mammalian and have different The of the pathway by may which is the pathway for anterograde cargo transport through the In to their role in proteins from the Golgi complex to the ER, COPI vesicles also have in retrograde transport within the Golgi In yeast, COPI vesicles are formed from the which carry one of or Each of these with the et al., et al., et al., the which also in with the et al., Because a of by the from the the retrograde pathway may be to all The secretory pathway at the of membranes that the This in and cells, it is of a compartment in cells, where it not as Proteins the TGN to one of the plasma membrane or the vacuole. However, some distinct and a protein vacuole et al., and 1999, and 1999, in this may be of these proteins in of proteins to the plasma membrane is believed to the proteins specific sorting This sorting is in the form of of in and specific in and (reviewed in and and it proteins to be sorted away from proteins destined for the plasma membrane. different vesicle types have to bud from the Each type a distinct protein coat, some of these proteins have (reviewed in and vesicles and the protein components of the coat have found in all and Battey et al., 1999, in this issue). consists of a and a that form a of and is recruited to membranes by a specific protein complex, called the complex distinct are found in complex is involved in formation of anterograde at the TGN, whereas a second is involved in formation of endocytic at the plasma membrane. The with specific protein on the of membrane proteins et al., and in so these proteins incorporated into the membrane of the important class of membrane proteins the cargo have from several and the mammalian the et al., and the sorting et al., et al., et al., the among these is such as a single the and a cargo proteins destined for the vacuole from intended for secretion is shown in Figure cargo proteins that sorting are in the TGN by the cargo The of the cargo is then bound by the complex The complex then the coat which in formation of the of into at the cargo proteins specific sorting that cargo for from the The of the cargo in to the sorting The of the cargo is then bound by the complex in The complex of cargo with by the of the coat drives of the for these carry cargo destined for the vacuole, from yeast, and that the PVC is the target for these vesicles In yeast, the PVC the et al., and transport of the complex to the PVC the the the as as the and et al., et al., et al., et al., also of et al., and and et al., 1996). also of which on the PVC et al., as as of and and et al., with the trafficking of some cargo in the cargo to be able to the complex and with on the PVC surface et al., these however, it that the trafficking in are to be more complex in some have more one type of vacuole et al., 1999, and and 1999, in this it that may have more one involved in In to et al., and et al., both with first as that a of the and the protein to the (i.e., the in the of et al., However, is more to and to it is to (see Figure that is found on the PVC of cells, where it with and is that on organelles (i.e., the PVC and the or different in different cell is to differentiate the role of the PVC and SNAREs in Endomembrane The endomembrane system consists of the TGN, the vacuole, the plasma membrane (PM), the PVC, and the endosome. components of are as with shown in formed at the TGN that carry the vesicles to the PVC, where with the PVC with coat bud from the TGN and to the vacuole in a that the for vesicles have coat and carry the and which with the plasma membrane and vesicles in mammalian use a distinct of SNAREs to cargo to the plasma it which type of SNAREs are in (see from the cell surface may use the and and the Retrograde transport from the PVC or endosome to the TGN also to depend on the that with the TGN The endosome may cargo destined for the vacuole by into the PVC (see A is for delivery of cargo from the PVC to the vacuole. In on the PVC of may reside on the in of the (see proteins are able to the PVC and from the TGN to the vacuole In yeast, proteins that this to the vacuole are packaged into vesicles and The complex is a complex that found in and et al., et al., not to and is to be of as a on et al., and vesicles to use a the and other components are for delivery of proteins to the vacuole by the pathway et al., for found for this pathway. Because it found in that and the are of in et al., it may be that the pathway also it that the mammalian coat may in some et al., The of complex in not some types of to more one of compartment et al., et al., et al., The vacuole is the compartment usually as being to a The second type of vacuole as a compartment for proteins that as and for the is believed that the protein are the vacuole with the vacuole, to degradation of the proteins and of Proteins to the protein vacuole are distinct from that are to the vacuole et al., 1996). and that some of these proteins are packaged into a type of coated a vesicle et al., 1996). The proteins destined for the have in membrane at the of in and through the Golgi complex before being at the TGN et al., et al., to a protein coat, the of this coat not A of proteins is their to into is that such may be the by which proteins are into the and is also that specific cargo in although the cargo is not found in et al., 1996). to the protein vacuole or first pass through the PVC is not that it be to determine in passes through a PVC distinct from that involved in trafficking of proteins to the vacuole. have from before their from the TGN et al., 1996). The of from other types of vesicles also in mammalian cells, where have found to bud from secretory et al., This may a for the recovery of cargo into is that the of the cargo may the packaging of the it may a to from the proteins are in The of these vesicles upon the of how cargo into distinct types of within the the TGN, proteins intended for secretion from the cell are packaged into type of vesicle. The pathway from the TGN to the cell surface is also important in for transport of cell to the cell surface, although it is not and proteins in the vesicle. of these vesicles in many cell although at a in vesicles have a protein coat, the of this coat not in system and from the TGN, secretory vesicles to use a for delivery of cargo at the plasma membrane In yeast, secretory vesicles carry the and et al., which with the plasma membrane and et al., et al., In mammalian cells, a distinct of and which in different cell to be involved in membrane For example, in cells, the and and the called are involved in the delivery of vesicles and other and several are believed to function in delivery of various cargo to the plasma membrane in other cell types et al., et al., mammalian have such a of SNAREs for membrane mammalian in secretion to the secretion of whereas others are In the cells, some proteins are from the plasma whereas distinct proteins are from the plasma membrane (reviewed in and with this particular are to specific of the plasma membrane of some mammalian cells, where may be involved in cargo destined for the plasma membrane away from cargo intended for the of the plasma membrane et al., 1996). delivery of cargo to the mammalian cell surface may a more that in to the mammalian have found in as and the of these function at the plasma membrane is of the mammalian and found to be to the plasma membrane and The other to function at the plasma membrane is in the cell in the and the protein is found at the of et al., et al., For this it is believed that to the vesicles that carry the and cell to form the cell that not function in trafficking of vesicles to the plasma membrane that it a specific role in vesicle trafficking cell also and secretion the of and although the of this pathway have not in of the anterograde secretory machinery through the retrograde pathway is essential to a endomembrane system. with retrograde transport from the vacuole or PVC, recovery of proteins from the cell surface by is to the various components of the a second role in the transport of proteins from the cell surface for in the vacuole, this is with the recycling of the retrograde as from at the plasma membrane first with the endosome. the PVC, the endosome is a sorting compartment that proteins intended for return to the cell surface from that are to be to the Golgi or to be degraded in the vacuole. In mammalian cells, many types of endosome have on the of several the type of cell in which For these various compartments are not instead, the term endosome is to all these (see In yeast, the retrograde as as to be by the and et al., shown in Figure to reside on the endosome and plasma traffic et al., The localization of is not as shown in Figure it is to the of the TGN in et al., this localization is the that a mammalian to is found on the Golgi complex et al., of are found in and Figure Figure that the PVC also a role in the endocytosed cargo that is destined for degradation in the vacuole et al., The compartments of the endomembrane system so that it can be to determine a particular endosome is involved in the anterograde or retrograde pathway. is also that some of the among the compartments of the secretory system not transport vesicles. may through fusion between compartments, whereas others may by of one compartment into For example, at endosome of yeast, and can both endocytosed and the as a of anterograde transport et al., et al., et al., of proteins to the plasma membrane or to the TGN et al., a endosome” proteins that are destined for degradation in the vacuole. This destined for degradation first must pass through the PVC and may so by et al., by the of the (i.e., or with the of the PVC (i.e., or Because the PVC is also a compartment, in which components are sorted to the TGN, the plasma and to the endosome and et al., it may a all that is destined for the vacuole. the which cargo from the PVC to the vacuole, may the is for a or may by fusion of these organelles and Figure the of compartments into one can be the of coat proteins for transport to and from these many compartments is more is also that and transport in different different the secretory system to be The of the secretory system are to be as the of the machinery are The of the of SNAREs in this However, it is that and have more SNAREs yeast, and although many of the in these to be (see and several that not have in have For example, mammalian and et al., et al., et al., whereas et al., and et al., In and to have forms of the single found in For example, are forms of in mammalian et al., et al., and forms of in both and and et al., all the SNAREs to a endomembrane and probably many more to their more is also that and mammalian The as it may to be to the of the secretory of it is that the of for vesicles and for target membranes is of The for example, also can be upon the vesicles et al., and the also is found on vesicles from the ER et al., may the that to to their of action or the packaging of the into retrograde vesicles. is also that the between compartments as of their which is by on the mammalian The protein of is somewhat to that of the PVC et al., 1996). However, is found at the TGN of mammalian cells, not at the PVC et al., is to a role in traffic from the TGN to the PVC, it found to be packaged into with the et al., this packaging into that is a it is more to The is that not can be packaged into a or In to their role in transport, the SNAREs may a role in organelle fusion of the vacuole, which small to form upon the between the and the et al., Other of fusion to In cells, for example, to disassembly of the membrane can be to be a of the and the Golgi stacks as of the Golgi complex between the the of the organelles must be This for both the ER and the mammalian Golgi complex. The ER is the et al., whereas the mammalian Golgi can be with just the et al., this not instead, it a called in et al., et al., this different of the or the for of the is also that the for vesicle trafficking in all For example, the with the on the to the Golgi complex and with the ER on the to the ER et al., et al., The to be more the with which it will found to be for delivery of cargo to the PVC through with the PVC also found to with the et al., This found also for the which with both and and In yeast, also with the TGN the and the et al., may not be the of targeting in the secretory system. The of that some form of before the by the in and mammalian that protein may function to vesicles at the appropriate membrane. For example, at the plasma membrane of cells, a protein complex called the is for delivery of secretory vesicles to the cell have that this complex of the plasma membrane et al., 1996). This complex also found in mammalian cells, where it may be involved in delivery of cargo to the of the plasma membrane et al., a complex called in the of vesicles at the Golgi complex et al., of the protein components of have in mammalian et al., that this complex be found in as a protein called in yeast, and in also in the of vesicles to the Golgi complex et al., et al., not to be of the complex et al., and and function or in to be Proteins that may function in of vesicles at the vacuole and and PVC et al., also have the for between the SNAREs, the in of SNAREs for transport in the secretory that some transport to the cell surface of mammalian may SNAREs et al., that and at the TGN with proteins that and that these then as a to the plasma membrane in a called and the role of in secretory vesicle targeting not be Each organelle in the endomembrane system a that is the of cargo vesicles that pass through each is that the of membranes can the of to assemble and et al., which many questions. the of vesicles different from that of the donor the of vesicles determine the of the target compartment, or is the of organelles by some other The in among the organelles can have on the and of the these are to be is that the of the organelles and the vesicles have a role just a for the cargo and a to the The and for on the is a of and is by from the and the of