Folding membrane proteins in vitro: a table and some comments.

Thirty-three years have elapsed since the first membrane protein (MP) was brought back in vitro to its native state starting from the completely unfolded polypeptide. Folding MPs is as useful from a practical point of view as it is thought-provoking from a theoretical one. Yet, this activity is considered as a high-risk, time-consuming endeavor, full of pitfalls, its path littered with the broken careers of graduate students sacrificed on the altar of a long shot that never paid off. In fact, a surprisingly high number of MPs have actually been folded or refolded in vitro. Analysis of the literature indicates (i) that the endeavor is not as desperate as it may seem, (ii) that techniques are diversifying and improving, and (iii) that many MPs do not need the cellular biosynthetic apparatus, nor even a membrane environment, to reach a functional 3D structure. A compilation, hopefully close to complete, is presented of MPs that have been (re)folded in vitro to-date, with the conditions of their synthesis, the denaturant(s) used, if any, and the (re)folding conditions, along with a few comments. The hope is that this analysis will encourage membrane protein biochemists to consider producing their target proteins in this way, help them decide about an experimental course, and stimulate the reflection about which environments favor membrane protein folding and why.