CALDENDRIN, A NOVEL NEURONAL CALCIUM-BINDING PROTEIN CONFINED TO THE SOMATO-DENDRITIC COMPARTMENT

Using antibodies against synaptic protein preparations, we cloned the cDNA of a new Ca2+-binding protein. Its C-terminal portion displays si gnificant similarity with calmodulin and contains two EF-hand motifs. The corresponding mRNA is highly expressed in rat brain, primarily in cerebral cortex, hippocampus, and cerebellum; its expression appears t o be restricted to neurons. Transcript levels increase during postnata l development. A recombinant C-terminal protein fragment binds Ca2+ as indicated by a Ca2+-induced mobility shift in SDS-polyacrylamide gel electrophoresis. Antisera generated against the bacterial fusion prote in recognize a brain-specific protein doublet with apparent molecular masses of 33 and 36 kDa, These data are confirmed by in vitro translat ion, which generates a single 36-kDa polypeptide, and by the heterolog ous expression in 293 cells, which yields a 33/36-kDa doublet comparab le to that found in brain. On two-dimensional gels, the 33-kDa band se parates into a chain of spots plausibly due to differential phosphoryl ation. This view is supported by in situ phosphorylation studies in hi ppocampal slices. Most of the immunoreactivity is detectable in cytosk eletal preparations with a further enrichment in the synapse-associate d cytomatrix. These biochemical data, together with the ultra-structur al localization in dendrites and the postsynaptic density, strongly su ggest an association with the somato-dendritic cytoskeleton, Therefore , this novel Ca2+-binding protein was named caldendrin.