Mixed neuron-glial cultures stained with CPCA-GAP43 (green) and rabbit antibody to alpha-II spectrin (RPCA-aII-Spec
, red), and DNA (blue). The GAP43 antibody stains numerous axonal and dendritic profiles in these cultures, clearly revealing the submembraneous cytoskeleton and vesicles.
GAP43 is very abundant protein which is found concentrated in neurons. One group discovered it as one of three proteins which becomes unregulated during the regeneration of the toad optic nerve (1).
Three GAPs (Growth associated proteins) were discovered, and the number 43 comes from the apparent SDS-PAGE molecular weight of the one named GAP43. The HGNC name for this protein is, not surprisingly, GAP43.
Later work showed that GAP43 does not run on SDS-PAGE in a fashion which accurately reflects its molecular weight, and that GAP43 proteins from different species may run at different apparent molecular weights. Partly due to these features the GAP43 protein was independently discovered by several different groups and therefore has several alternate names, such as protein F1, pp46, neuromodulin, neural phosphoprotein B-50 and calmodulin-binding protein P-57. In each case the number reflects the apparent SDS-PAGE molecular weight, and underlines the unusual properties of this molecule.
Mammalian GAP43 proteins contain only 226-243 amino acids, and so the real molecular weight is 23.61-25.14 kDa (to perform such calculations yourself, see this link).
GAP43 is one of many highly negatively charged extended molecules which lack a well defined tertiary structure and run anomalously on SDS-PAGE. Other examples are microtubule associated proteins MAP2, tau and the Neurofilament subunits. GAP43 has been extensively studied and is known to be a major protein kinase C substrate and to bind calmodulin avidly. GAP43 is anchored to the plasma membrane by palmitoylation modifications.
The antibody was raised against the C-terminal peptide of rat and mouse GAP43, which is KEDPEADQEHA, to which an N terminal Cysteine residue was added to allow chemical coupling to Keyhole Limpet Hemocyanin carrier protein. The corresponding C-terminal peptide in the human GAP43 sequence is KGEEREADQEHA, which is similar enough that our antibody binds to this also.