Antigen presentation

Reorganization of multivesicular bodies regulates MHC class II antigen presentation by dendritic cells. Kleijmeer, M. et al. J. Cell Biol. 155, 53?63 (2001) [PubMed]

The maturation of dendritic cells ? antigen-presenting cells that initiate primary T-cell responses ? is characterized by the redistribution of MHC II molecules from the internal membranes of multivesicular bodies to the external membranes of these organelles. There, MHC II encounters DM, an accessory molecule required for peptide loading. The mechanism through which mixing of internal and external membranes occurs is not well understood, but this relocalization could prove important in regulating loading of MHC II antigens.

Cell cycle

The human decatenation checkpoint. Deming, P. B. et al. Proc. Natl Acad. Sci. USA 98, 12044?12049 (2001) [PubMed]

Cells actively delay mitosis until daughter chromatids have become decatenated after DNA replication. Here, Kaufmann and colleagues provide evidence for a decatenation checkpoint. Cells that expressed an ataxia-telangiectasia mutated- and rad3-related (ATR) kinase-inactive allele, or BRCA1-mutant cells, failed to undergo mitotic delay ? which seems to require nuclear exclusion of cyclin B1/Cdk1 complexes ? implicating ATR-mediated signalling and BRCA1 in this G2 checkpoint.

Ion channels

A sperm ion channel required for sperm motility and male fertility. Ren, D. et al. Nature 413, 603?609 (2001) [PubMed]

Clapham and colleagues describe the cloning of CatSper, a six-transmembrane-spanning cation channel localized to the principal piece of the sperm tail. Sperm from CatSper−/− mice couldn't elicit a Ca2+ influx in response to either cAMP or cGMP, implying that CatSper comprises a cyclic nucleotide-gated Ca2+-permeant channel. CatSper functions in male fertility, as sperm from mutant mice were poorly motile and could not penetrate the outer layers of the egg.

Signal transduction

Carboxyl-terminal modulator protein (CMTP), a negative regulator of PKB/Akt and v-Akt at the plasma membrane. Maira, S.-M. et al. Science 294, 374?380 (2001) [PubMed]

Protein kinase B/Akt has well-established functions in insulin signalling, cell survival and transformation. Searching for new PKB/Akt interactions, Hemmings and colleagues identified CTMP ? carboxy-terminal modulator protein. By binding to the carboxy-terminal regulatory domain of PKBα at the plasma membrane, CTMP can inhibit phosphorylation of PKB on serine 473 and threonine 308, and thereby negatively regulate PKB activity.