Pilings in a lipid sea

Pilings in a lipid sea T etraspanin contains a tightly packed quartet of transmembrane helices, according to a new, high resolution electron microscopy structure deduced by Min et al. (page 975). The rigid tetraspanin proteins may thus act as stable pilings in a lipid sea, say the authors. Tetraspanins associate with a number of important transmembrane proteins such as integrins to form distinct signaling networks, called tetraspanin webs. Lipids trapped in the networks create microdomains with characteristic compositions and unique properties. The web under study here was made up of uroplakins. Two uroplakin tetraspanins each pair with a single transmembrane partner forming a heterotetramer subunit, six of which then form a 16-nm wide, ring-shaped particle. A two-dimensional crystalline array of these particles contributes to a remarkable urothelial permeability barrier, which keeps urine on one side and body fl uid on the other. These arrays are particularly suitable for electron microscopic studies. At 6 Å resolution, the team could assign secondary structures to certain regions of the particle. The angle between membrane-traversing α-helices is minimal, so that the helices can pack tightly together. Each single transmembrane partner is shaped like an L that covers the tetraspanins and connects to a neighboring subunit. The relatively rigid tetraspanin structure is ideal for docking other tall signaling transmembrane proteins. Tetraspanins can also help these proteins to pass messages into the cell, and are themselves the receptors and signaling conduits for some bacteria and viruses. Future structural studies should reveal how these signals are transduced to trigger a wide variety of cellular responses. half bridge. Finally, the group shows that Sfi 1p's NH 2 terminus is at the SPB whereas the COOH terminus is at the far end of the half bridge (or the center of the structure once the half bridge duplicates). Kilmartin suggests that the half bridge duplicates when an unknown signal causes two copies of Sfi 1p to link to each other via their COOH termini. Then the NH 2 terminus of the new Sfi 1p nucleates the growth of the new SPB. The signals controlling these events are not known. The test of this model is to see whether an Sfi 1p with fewer repeats will make SPBs with shorter half bridges. A straight rod of Sfi 1p appears to act as a simple building block that aids the doubling of the budding yeast spindle pole body (SPB), say Li et …