Membrane Protein Insertion in Cells: Principles, Pathways, and Quality Control.

Integral membrane proteins comprise at least a quarter of every proteome. To fold and function properly, these proteins must insert into the correct membrane with the proper topology and orientation. Although their transmembrane helices are chemically compatible with the bilayer, insertion inside cells is not a simple spontaneous event. It must occur in a crowded environment, at the correct membrane, and it often involves challenges such as transferring hydrophilic segments across the bilayer or accommodating helices that are only marginally compatible with it. Cells therefore rely on dedicated systems that direct membrane proteins to the membrane, mediate their insertion, and monitor the process to ensure that it occurs correctly. This review outlines the principles that govern membrane protein insertion in cells and explains how transmembrane sequence features interact with the machineries that mediate their entry into the bilayer. We highlight the major insertion systems of bacteria and eukaryotes and the auxiliary factors that support them, and describe how these pathways accommodate the broad range of membrane protein architectures found in cells, from single-pass proteins to complex multispanning transporters. We also discuss how cells maintain accuracy when insertion fails, through mechanisms that detect and resolve misinsertion. Together, these concepts present membrane protein insertion as a coordinated, adaptable, and safeguarded process, shaped by the interplay between sequence properties, membrane environments, and the machinery responsible for building the membrane proteome.