PHOSPHORYLATION OF THE IQ DOMAIN REGULATES THE INTERACTION BETWEEN CA2-VECTOR PROTEIN AND ITS TARGET IN AMNPHIOXUS()

Calcium vector protein target (CaVPT) a 26-kDa endogenous target of ca lcium vector protein from Amphioxus (CaVP), contains three distinct re gions: a N-terminal Pro-Ala-Lys-rich motif segment 36-50 displaying se quence similarity to the calmodulin-binding site in neuromodulin and n eurogranin where they are designated as the IQ domain; and two immunog lobulinlike folds. The phosphorylation by protein kinase C of Ser-43 i n the IQ domain drastically decreases the affinity of CaVPT for CaVP a nd CaVP protects CaVPT from phosphorylation. Phosphorylation by the ca talytic subunit of cyclic AMP-dependent protein kinase has a similar e ffect, but in addition to Ser 43 four other phosphorylated sites were identified. Removal of the Pro-Ala-Lys-rich region and the IQ domain i n CaVPT by trypsin leads to the loss of binding to CaVP, whereas the c hymotryptic fragment, containing these regions and first immunoglobuli n-like domain, retained the ability to interact with CaVP. A synthetic IQ domain alone interacts strongly with calmodulin, but not with CaVP . Two main conclusions can be drawn from this study: 1) the regulation of interaction between CaVP and CaVPT is very similar to the mechanis m observed in the complex between neuromodulin or neurogranin and calm odulin; 2) in spite of this similarity the entire CaVP-binding site is not restricted to the IQ domain; in addition the Pro-Ala-Lys-rich mot if may he necessary for high affinity binding to CaVP.