In striated muscles, invaginations
from the plasma membrane, termed
transverse tubules (T-tubule), function in the excitation–contraction
coupling machinery. BIN1 (isoform8) plays a critical role in the biogenesis
of T-tubules. BIN1 contains an N-terminal BAR domain to sense and
induce membrane curvature, an isoform8-specific polybasic motif (exon10)
as the phosphoinositide binding module and a C-terminal Src homology
3 (SH3) domain for the recruitment of downstream proteins such as
dynamin 2. Previous studies of N-BAR domains focused on elucidating
mechanisms of membrane curvature sensing and generation (MC-S&G).
Less is known about how MC-S&G is regulated. We found that the
SH3 domain binds to the exon10 motif more strongly compared to the
proline-rich domain (PRD) of dynamin 2. Furthermore, we found that
the MC-S&G ability of full-length BIN1 is inhibited on membranes
lacking PI(4,5)P2. Addition of PI(4,5)P2 in
the membrane activates BIN1 to sense and induce membrane curvature.
Co-presence of the SH3 domain and exon10 motif leads to the strongest
phosphoinositide-mediated control of BIN1 function. Addition of SH3
domain ligand (such as PRD peptides), as well as addition of the water-soluble
PI(4,5)P2 analogue, can both enhance the MC-S&G ability
of BIN1 on membranes without PI(4,5)P2, indicating that
the key to activate BIN1 is to disrupt the exon10–SH3 interaction.
The nonsense mutation K436X, found in centronuclear myopathy (CNM)
patients, abolishes SH3 domain binding with either exon10 or the PRD
motif, resulting in increased membrane deformation capacity. Our results
suggest an autoinhibition model for BIN1 that involves a synergistic
regulation by membrane composition and protein–protein interactions.