Size, shape, and solubility of a class of releasable cell surface proteins of sympathetic neurons.

Exposed on the cell surface of sympathetic neurons in culture is a family of high molecular weight glycoproteins (B1, B2, B3, and B4, related to the NILE protein) which undergoes post-translational modification (Sweadner, K. J. (1983) J. Neurosci. 3: 2504-2517). B1 and B3 are converted to B2 and B4 by what might be limited proteolysis. These proteins normally require detergents to release them from the cells. When neurotransmitter release is evoked chemically, however, derivatives of the proteins (S2 and S4) are released into the medium. A hydrodynamic analysis of the structure of the released proteins and their membrane-associated precursors was undertaken to determine whether the proteins are released as membrane fragments, aggregates, or monomers in solution, and to give information on the structure and disposition of the proteins on the cell surface. Measurements of the Stokes radii, sedimentation coefficients, partial specific volumes, and frictional coefficients of the proteins indicate that they are released into the medium as soluble monomers. The hydrodynamic analysis also indicates that they are nonglobular (probably fibrous) in shape, both before and after post-translational modification and release. Their true molecular weights are calculated to be approximately 130,000 to 170,000. Although B1, B2, B3, and B4 are probably intrinsic membrane proteins, their releasability suggests that most of their mass is exposed to the aqueous extracellular medium.