Morphogenesis of the endoplasmic reticulum: beyond active membrane expansion.

The endoplasmic reticulum (ER) of higher eukaryotic cells is a dynamic network of interconnected membrane tubules that pervades almost the entire cytoplasm. On the basis of the morphological changes induced by the disruption of the cytoskeleton or molecular motor proteins, the commonly accepted model has emerged that microtubules and conventional kinesin (kinesin-1) are essential determinants in establishing and maintaining the structure of the ER by active membrane expansion. Surprisingly, very similar ER phenotypes have now been observed when the cytoskeleton-linking ER membrane protein of 63 kDa (CLIMP-63) is mutated, revealing stable attachment of ER membranes to the microtubular cytoskeleton as a novel requirement for ER maintenance.

Phosphorylation controls CLIMP-63-mediated anchoring of the endoplasmic reticulum to microtubules.

The microtubule-binding 63-kDa cytoskeleton-linking membrane protein (CLIMP-63) is an integral membrane protein that links the endoplasmic reticulum (ER) to microtubules. Here, we tested whether this interaction is regulated by phosphorylation. Metabolic labeling with (32)P showed that CLIMP-63 is a phosphoprotein with increased phosphorylation during mitosis.

Two related subpellicular cytoskeleton-associated proteins in Trypanosoma brucei stabilize microtubules.

The subpellicular microtubules of the trypanosome cytoskeleton are cross-linked to each other and the plasma membrane, creating a cage-like structure. We have isolated, from Trypanosoma brucei, two related low-molecular-weight cytoskeleton-associated proteins (15- and 17-kDa), called CAP15 and CAP17, which are differentially expressed during the life cycle. Immunolabeling shows a corset-like colocalization of both CAPs and tubulin.