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Endocytic properties of the M-type 180-kDa receptor for secretory phospholipases A2 (sPLA2) were first investigated in rabbit myocytes that express it at high levels. Internalization of the receptor was shown to be clathrin-coated pit-mediated, rapid (ke = 0.1 min-1), and ligand-independent. The signal sequence for internalization was then identified upon transient and stable expression of various receptor constructs with mutated cytoplasmic sequences. Analysis of the internalization efficiency of the mutants suggested that the NSYY motif encodes the major endocytic signal, with the distal tyrosine residue playing the key role. Amino acid substitutions at the putative casein kinase II phosphorylation site of the receptor did not affect internalization. A chimeric protein composed of the extracellular and transmembrane domains of the rabbit sPLA2 receptor and of the cytoplasmic domain of the structurally homologous human macrophage mannose receptor retained the high affinity for sPLA2 and was internalization competent, exhibiting 50% endocytic activity of the M-type sPLA2 receptor. The results indicate the compatibility of the structural domains of the two parent proteins and provide evidence for the interchangeable character of their internalization signals.
Zvaritch et al. (Mon,) studied this question.