Ca²⁺ ‐binding proteins and contractility of the infraciliary lattice in Paramecium

Summary— The infraciliary lattice (ICL) is the innermost cortical cytoskeletal network of Paramecium . Its meshes which run around the proximal end of basal bodies form a continuous contractile network beneath the cell surface. We had previously shown that the network, which could be recovered in a contracted form and selectively solubilized by EGTA from an ICL‐enriched cell fraction, was principally composed of 23–24 kDa polypeptides cross‐reacting with antibodies raised against the 22 kDa Ca 2+ ‐binding proteins of the ecto‐endoplasmic boundary (EEB), a contractile cytoskeletal network of another ciliate Isotricha prostoma . We show here 1) that the ICL also comprises a 220 kDa polypeptide; 2) that the 23–24 kDa polypeptides are resolved in 2D gels into 11 spots of acidic pI, 7 of which are both Ca 2+ ‐binding and cross‐reacting with the anti EEB polypeptides; 3) that the network displays a high Ca 2+ ‐affinity as the treshold for solubilization/co‐precipitation of both high and low MW polypeptides is around 10 −8 M free Ca 2+ ; 4) that in vivo contraction of the network occurs upon physiological increase of internal calcium concentration. The likely phylogenetic relationships of the 23–24 kDa ICL polypeptides with the calmodulin related family of Ca 2+ ‐modulated polypeptides and the functions of the ICL in cell contractility and Ca 2+ homeostasis are discussed.