GRAVIN, AN AUTOANTIGEN RECOGNIZED BY SERUM FROM MYASTHENIA-GRAVIS PATIENTS, IS A KINASE SCAFFOLD PROTEIN

Background: Subcellular targeting of protein kinases and phosphatases provides a mechanism for co-localizing these enzymes with their prefer red substrates. A recently identified mammalian scaffold protein, AKAP 79, controls the location of two broad-specificity kinases and a phosp hatase. Results: We have identified and characterized another mammalia n scaffold protein which coordinates the location of protein kinase A and protein kinase C. We isolated a cDNA encoding a 250 kDa A-kinase a nchoring protein (AKAP) called gravin, which was originally identified as a cytoplasmic antigen recognized by myasthenia gravis sera. Sequen ce homology to proteins that are known to bind protein kinase C sugges ts that gravin also binds this kinase. Studies of binding in vitro sho w that residues 1526-1780 of gravin bind the regulatory subunit (RII) of protein kinase A with high affinity, and residues 265-556 bind prot ein kinase C. Gravin expression in human erythroleukemia cells can be induced with phorbol ester, resulting in the detection of a 250 kDa RI I- and PKC-binding protein. Immunolocalization experiments show that g ravin is concentrated at the cell periphery and is enriched in filopod ia. Gravin staining is coincident with an AKAP detected by an in situ RII-overlay assay, and a PKA-gravin complex can be isolated from human erythroleukemia cells. Conclusions: We present biochemical evidence t hat gravin forms part of a signaling scaffold, and propose that protei n kinases A and C may participate in the coordination of signal transd uction events in the filopodia of human erythroleukemia cells.