Expression, characterization and crystallization of SALM family adhesion proteins

Abstract

Synaptic adhesion-like molecule (SALM) family which comprises five members; SALMs 1–5, are located in synaptic membranes and participate mainly in synaptogenesis and neurite outgrowth. SALM3 and SALM5 have been known to interact with presynaptic n tyroleukocyte common antigen-related receptor protein phosphatases (LAR-RPTPs) differentially. We purified SALM1 and SALM5 and used thermofluor assay, negative staining electron microscopy (negative staining EM), and multi-angle laser light scattering (MALLS) to characterize them. We also screened for crystallization conditions for both the proteins. Furthermore, we tested SALM5 interaction with different splice variants of protein tyrosine phosphatase receptor ϭ (PTPRϭ) using enzyme-linked immune sorbent assay (ELISA). We found that both the SALMs exist in dimeric form. In addition, we deciphered the elusive high order oligomeric form of SALM1. Different crystalline forms of SALM5 were obtained and the best resolution of diffraction obtained was 3.6 Å. No further improvement in SALM1 crystals’ morphology was obtained during optimization of crystallization conditions for the protein. SALM5 did not bind PTPRϭ wild type variant but bound PTPRϭ meB variant with low affinity. In sum, the study deciphered high order oligomeric structure of SALM1 and opened the way towards elucidating crystal structures of SALM5 with further optimization of crystallization conditions.