ABSTRACT
Secretoglobin (SCGB) 3A2 belongs to an intriguing family of small, secreted proteins present only in mammals. Although members of the SCGB protein family have distinct amino acid sequences, they share structural similarities. Of particularly interest is the not yet fully understood self-assembly ability of SCGBs, which arise from covalent disulfide dimerization and non-covalent oligomerization. Recently, SCGB3A2 has attracted attention for its singular expression profile in airways. However, the knowledge on SCGB3A2 (patho)physiology derives exclusively from in vivo and complex ex vivo mixtures, which hampers characterization of the mechanisms driving SCGB3A2 structural behavior. Herein, we document the chemical synthesis of SCGB3A2 in multi-milligram quantities. Key to access both monomeric and homodimeric SCGB3A2 analogues was the use of KAHA ligation and enabled masking of the cysteine residue. The synthetic proteins were used to investigate the SCGB3A2 self-assembly profile, confirming their high propensity to dimerization even in the absence of the key Cys residue.
Subject(s)
Dimerization , Humans , Protein Multimerization , Photochemical ProcessesABSTRACT
Inhibitor of apoptosis proteins (IAPs) bind to pro-apoptotic proteases, keeping them inactive and preventing cell death. The atypical ubiquitin ligase BIRC6 is the only essential IAP, additionally functioning as a suppressor of autophagy. We performed a structure-function analysis of BIRC6 in complex with caspase-9, HTRA2, SMAC, and LC3B, which are critical apoptosis and autophagy proteins. Cryo-electron microscopy structures showed that BIRC6 forms a megadalton crescent shape that arcs around a spacious cavity containing receptor sites for client proteins. Multivalent binding of SMAC obstructs client binding, impeding ubiquitination of both autophagy and apoptotic substrates. On the basis of these data, we discuss how the BIRC6/SMAC complex can act as a stress-induced hub to regulate apoptosis and autophagy drivers.
