Sense, plug, and seal: proteins as both rapid responders and constitutive barriers supporting organelle compartmentalization.

Although organellar compartmentalization is primarily established by the delimiting phospholipid bilayer membranes, the contribution of proteins has been less appreciated. Recently, studies across many realms of cell biology have put new focus on the role of proteins in acting as diffusion barriers in contexts where there are constitutive, regulated, or pathological discontinuities in membranes. Here, we synthesize longstanding observations of proteins acting as both barriers to lateral diffusion on membranes and diffusion in three-dimensional space. In particular, we focus on an emerging, conserved two-step paradigm of protein diffusion barriers that rapidly assemble in response to membranous organelle damage: a first phase of coincident sensing and stopgap "plugging" by responding repair proteins followed by a second phase of membrane sealing. We highlight recent work exemplifying this sense, plug, and seal paradigm at the postmitotic nuclear envelope and at ruptures of the interphase nuclear envelope, lysosomes, and the plasma membrane. Taken together, we highlight how cells use a variety of constitutive and induced proteinaceous barriers that support the role of biological membranes in defining organelle compartmentalization. Determining the biophysical nature of these barriers, and their means of "sensing" membrane rupture, will be an exciting avenue of future investigations.