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Binding Chitin

Chitin (green) is deposited in the cell wall in several locations during the cell cycle.

In yeast, chitin is laid down at three locations in the cell wall during the cell cycle: in a ring at the mother-bud neck, in the partition that forms between mother and daughter cells at cytokinesis, and in the cell wall of the daughter cell. Chitin is bound to the cell wall structure by two types of linkages: to the non-reducing end of (1–3)glucan and to side chains of (1–6)glucan. Enrico Cabib and Angel Durán hypothesized that the chitin in the ring at the mother-daughter neck might be linked differently from the chitin that dispersed throughout the cell wall.

To test their hypothesis, Cabib and Durán radiolabeled chitin in unbudded or early budding cells, cross-linked it, and analyzed it by differential digestion with (1–3)- or (1–6)glucanases followed by fractionation of the digestion products. To perform these experiments, the authors devised novel techniques necessary to solubilize and analyze free and cross-linked chitin. Their results confirmed that the wall chitin is primarily linked to (1–6)glucan whereas that in the ring is linked to (1–3)glucan. Because chitin and (1–6)glucan compete for the same attachment site on (1–3)glucan, a chitin cap on (1–3)glucan non-reducing ends makes them unavailable for cell wall growth. These results provide significant new information about both the regulation of cell wall biosynthesis in yeast and the fundamental process with which a cell can remodel a structure at a defined location.

FOOTNOTES

See referenced article, J. Biol. Chem. 2005, 280, 9170-9179

RAMP3 Regulates Receptor Recycling

RAMP3 RNA interference results in lack of receptor recycling.

Three receptor activity-modifying proteins (RAMPs 1, 2, and 3) are required for plasma membrane expression and ligand binding of several G-protein-coupled receptors. For example, if RAMP1 binds to the calcitonin receptor-like receptor (CRLR), then a cell surface calcitonin generelated peptide 1 receptor is produced. In contrast, the binding of RAMP2 or -3 to CRLR produces an adrenomedullin peptide receptor. Jennifer M. Bomberger and colleagues now show that in addition to these roles, RAMP3 binding to the CRLR is required for recycling of internalized adrenomedullin receptors.

The intracellular itineraries of G-protein-coupled receptors can be influenced by interactions between a PDZ domain located in the carboxyl terminus of the receptor and other proteins. In some cases, these interactions can change the fate of a receptor from degradation to salvation in the form of recycling to the cell surface. Because RAMP3 contains a PDZ domain but RAMP1 and RAMP2 do not, Bomberger et al. hypothesized that RAMP3 might regulate the trafficking of CRLR. They found that coexpression of a protein known to influence the movement of other receptors (the N-ethylmaleimide-sensitive factor or NSF) with the CRLR-RAMP3 complex but not the CRLR-RAMP1 or CRLR-RAMP2 complexes changed receptor trafficking from a degradative to a recycling pathway. This change required the interaction of the RAMP3 PDZ domain with NSF. An increasing body of evidence indicates that interacting or modifying proteins play important roles in the functions of G-protein-coupled receptors. The present paper identifies RAMP3 as an important component of adrenomedullin receptor biology, and inasmuch as sequence comparisons show that 30% of G-protein-coupled receptors contain PDZ domains, it serves as a harbinger of exciting future results.

FOOTNOTES

See referenced article, J. Biol. Chem. 2005, 280, 9297-9307