p36, the major cytoplasmic substrate of src tyrosine protein kinase, binds to its p11 regulatory subunit via a short amino‐terminal amphiphatic helix.

Protein I is a hetero‐tetramer which contains two copies each of p11 and p36. p36 (calpactin I, lipocortin II) is a major substrate of retrovirally encoded tyrosine protein kinases, while p11 modulates several Ca2+‐induced properties also displayed by p36 alone. Here we have characterized the p11 binding site on p36 by fluorescence spectroscopy using porcine p36 labelled at cysteine 8 with the fluorophore Prodan (6‐proprionyl‐2‐dimethylamino‐naphthalene). We have used peptides of differing length from the amino‐terminal domain of p36 to restrict the major binding site to the first 12 residues. Noticeable binding is still observed with a peptide containing only the first nine residues. Interestingly the N‐terminal acetyl group of p36 forms a functional part of the p11 binding site. CD studies indicate that the binding region can form an alpha‐helix, which seems to have amphiphatic properties when projected on a helical wheel. This structural element is also known for a calmodulin binding protein. Thus the question is raised whether other p11/calmodulin‐related proteins interact with their target proteins via a similar mechanism. We also discuss how p11 could modulate p36 associated properties.