P-57 is a neural specific calmodulin-binding protein

P-57 is a novel calmodulin-binding protein which has recently been isolated from bovine cerebral cortex (Andreasen, T. J., Luetje, C. W., Heideman, W., and Storm, D. R. (1983) Biochemistry 22, 4615-4618). In contrast to all other calmodulin-binding proteins characterized thus far, P-57 has equivalent or higher affinity for calmodulin in the absence of free Ca2+ compared to the presence of Ca2+. In this study, the distribution of P-57 in other tissues and within brain was examined using a radioimmune assay and photoaffinity labeling with azido-125I-calmodulin. P-57 was not found in tissues other than brain, retina, and spinal cord. Within brain, P-57 levels varied from 0.1% of the total protein in white matter regions to about 0.5% in cell body-rich fractions. The protein was found in both membrane and soluble fractions. P-57 is the most abundant calmodulin-binding protein in brain and appears to be neural specific. The concentrations of P-57 in brain and its affinity for calmodulin in the absence of Ca2+ are sufficient to complex a significant fraction of the total calmodulin present.