Transmembrane and secretory proteins are made on polysomes attached to the membranes of the ER. Individual ribosomes within the polysomes are attached to the ER by translocon complexes (TCs), which contain numerous transmembrane proteins. The lateral movement of TCs within ER membranes is severely restricted. How is this achieved? The answer, suggest Gert Kreibich and colleagues, is CLIMP-63-mediated binding of microtubules to the ER (see ). Microtubules are often seen near the ER, and CLIMP-63 is one of the proteins thought to mediate the interaction between these two structures. The authors use fluorescence recovery after photobleaching (FRAP) to show that breakdown of microtubules induced by drugs or overexpression of the microtubule-severing protein spastin increases the lateral mobility of TCs. Knocking down CLIMP-63 by RNAi also greatly increases TC lateral mobility. Thus, propose the authors, the CLIMP-63-mediated interaction between microtubules and the ER might immobilise TCs within membrane-bound polysomes and could help to segregate the rough and smooth ER.
