PONTICULIN IS AN ATYPICAL MEMBRANE-PROTEIN

We have cloned and sequenced ponticulin, a 17,000-dalton integral memb rane glycoprotein that binds F-actin and nucleates actin assembly. A s ingle copy gene encodes a developmentally regulated message that is hi gh during growth and early development, but drops precipitously during cell streaming at similar to 8 h of development. The deduced amino ac id sequence predicts a protein with a cleaved NH2-terminal signal sequ ence and a COOH-terminal glycosyl anchor, These predictions are suppor ted by amino acid sequencing of mature ponticulin and metabolic labeli ng with glycosyl, anchor components. Although no alpha-helical membran e-spanning domains are apparent, several hydrophobic and/or sided beta -strands, each long enough to traverse the membrane, are predicted. Al though its location on the primary sequence is unclear an intracellula r domain is indicated by the existence of a discontinuous epitope that is accessible to antibody in plasma membranes and permeabilized cells , but not in intact cells, Such a cytoplasmically oriented domain also is required for the demonstrated role of ponticulin in binding actin to the plasma membrane in vivo and. in vitro (Hitt, A. L., J. H. Hartw ig, and E. J. Luna. 1994. Ponticulin is the major high affinity link b etween the plasma membrane and the cortical actin network in Dictyoste lium. J. Cell Biol. 126:1433-1444). Thus, ponticulin apparently repres ents a new category of integral membrane proteins that consists of pro teins with both a glycosyl anchor and membrane-spanning peptide domain (s).