Gravin, an autoantigen recognized by serum from myasthenia gravis patients, is a kinase scaffold protein

Subcellular targeting of protein kinases and phosphatases provides a mechanism for co-localizing these enzymes with their preferred substrates. A recently identified mammalian scaffold protein, AKAP79, controls the location of two broad-specificity kinases and a phosphatase.

We have identified and characterized another mammalian scaffold protein which coordinates the location of protein kinase A and protein kinase C. We isolated a cDNA encoding a 250 kDa A-skinase anchoring protein (AKAP) called gravin, which was originally identified as a cytoplasmic antigen recognized by myasthenia gravis sera. Sequence homology to proteins that are known to bind protein kinase C suggests that gravin also binds this kinase. Studies of binding in vitro show that residues 1526-1780 of gravin bind the regulatory subunit (RII) of protein kinase A with high affinity, and residues 265-556 bind protein kinase C. Gravin expression in human erythroleukemia cells can be induced with phorbol ester, resulting in the detection of a 250 kDa RII- and PKC-binding protein. Immunolocalization experiments show that gravin is concentrated at the cell periphery and is enriched in filopodia. 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.

We present biochemical evidence that gravin forms part of a signaling scaffold, and propose that protein kinases A and C may participate in the coordination of signal transduction events in the filopodia of human erythroleukemia cells.