Structural and functional relationship of red blood cell protein 4.1 to synapsin I

It has been suggested that the neuron specific protein synapsin I is closely related to red blood cell (rbc) protein 4.1. A systematic comparison of the structural and functional properties of rbc protein 4.1 and synapsin I has been carried out. There is approximately a three order of magnitude difference in cross reactivity of synapsin I with rbc 4.1 antiserum vs. synapsin I antiserum, as determined by a competitive quantitative dot assay. Two-dimensional chymotryptic iodopeptide mapping analysis demonstrated limited peptide homology (~34% spot overlap) between rbc 4.1 and synapsin I. Dephosphorylated synapsin I binds saturably to brain spectrin (240/235) with an estimated dissociation constant (K(d)) of 700 nM and a maximal binding capacity of 4 mol synapsin I/mol spectrin tetramer, similar to the affinity and stoichiometry of 4.1 binding to rbc spectrin. Synapsin I was found to bind to the terminal ends of the brain spectrin tetramer by low-angle rotary shadowing, analogous to 4.1 binding to rbc spectrin. In summary, synapsin I is structurally and immunologically distinct from rbc 4.1, yet shares functional similarities with rbc 4.1 with respect to its spectrin binding characteristics.