BCAP31 Full Name
B-cell receptor-associated protein 31
BCAP31 Introduction
B-cell receptor-associated protein 31 (BCAP31), commonly abbreviated as BAP31, is a protein that plays a crucial role in cell biology. BAP31 itself is an integral membrane protein with a molecular weight ranging from approximately 28 to 31 kDa. Structurally, it features a distinctive topology consisting of three transmembrane helices, which divide the protein into two luminal domains facing the endoplasmic reticulum lumen and two cytosolic domains exposed to the cytoplasm. This complex architecture allows it to function as a multifunctional platform on the ER membrane, interacting with a variety of intracellular and extracellular signaling molecules and proteins. The gene encoding BAP31 has multiple transcripts and splice variants, suggesting it may be tightly regulated and serve diverse functions in different tissues or cellular states. As a widely expressed protein across various tissues, BAP31 is an indispensable component in maintaining fundamental cellular processes, especially in managing protein life cycles and determining cell fate.
The core function of BAP31 is centered on the ER, a key organelle responsible for protein synthesis, folding, modification, and transport. It recognizes and binds to newly synthesized transmembrane proteins such as major histocompatibility complex class I molecules (MHC class I), cystic fibrosis transmembrane conductance regulator (CFTR), immunoglobulin D, and cytochrome P450, guiding them correctly from the ER into the secretory pathway. In this process, BAP31 recruits other chaperone proteins (e.g., Sec61β) to assist in the proper folding and assembly of these client proteins. When proteins misfold, BAP31 switches roles and participates in initiating the endoplasmic reticulum-associated degradation (ERAD) pathway. It recognizes defective proteins and directs them to degradation complexes for eventual clearance via the ubiquitin-proteasome system, thereby preventing toxic protein accumulation and maintaining cellular homeostasis. Secondly, BAP31 serves as a key regulatory node in the apoptosis signaling pathway. Upon stimulation by apoptotic signals, intracellular caspases—especially caspase-8—are activated and specifically cleave BAP31 to generate a 20 kDa pro-apoptotic fragment, p20Bap31. This p20 fragment exhibits potent biological activity, directly acting on the ER to induce the release of calcium ions (Ca2+) from ER stores. The released calcium is taken up in large amounts by nearby mitochondria, triggering mitochondrial dysfunction, promoting Drp1-dependent mitochondrial fission, and ultimately—through caspase-8-mediated signal amplification—leading to the release of cytochrome c from mitochondria into the cytoplasm. This activates downstream caspase cascades and executes the cell death program. Additionally, the full-length BAP31 protein can form complexes with anti-apoptotic proteins such as Bcl-2/Bcl-XL, connecting pro-apoptotic initiators (e.g., procaspase-8) with inhibitory signals (Bcl-2 family) on a finely tuned regulatory platform that determines cell survival or death.
Figure 1. Function of BAP31 at MAMs. (Source: Quistgaard EM. 2021)
Disease caused by loss-of-function mutations in the BCAP31 gene is an X-linked recessive syndrome, clinically referred to as "BCAP31-associated syndrome" or named by its main features, such as Deafness, Dystonia, and Optic Neuropathy (DDON) syndrome. Since the gene is located on the X chromosome, male patients (who possess only one X chromosome) typically exhibit severe, classic clinical symptoms, while female heterozygous carriers may be asymptomatic or show mild and variable manifestations, partly depending on the pattern of X-chromosome inactivation.
Alternate Names for BCAP31
BCAP31
B-cell receptor-associated protein 31
CDM
DDCH
BAP31
6C6-AG
DXS1357E
p28 Bap31
BCR-associated protein Bap31
6C6-AG tumor-associated antigen
