Expression of a dominant-negative myosin-1A (green) results in loss of apical SI (red).

A monomeric myosin first spotted in electron micrographs almost 30 years ago has finally been united with a proposed function. Tyska and Mooseker (page 395) report that myosin-1A associates with and anchors a raft component in apical membranes.

Myosin-1A was previously thought to shuttle Golgi-derived cargos to the plasma membrane (after most of the distance had been covered using microtubule-based motors). But the in vitro evidence for this came from undifferentiated cells, and in mature, polarized colon epithelial cells, Tyska and Mooseker see no evidence of shuttling by myosin-1A. What they did spot was cofractionation and cross-linking of myosin-1A with the transmembrane disaccharidase sucrase-isomaltase (SI). This raft protein is lost from the apical surface when a fragment of either myosin-1A or SI interferes with the link between the two full-length proteins.

Thus, myosin-1A may serve as an anchor, with the clustering of SI in rafts helping to secure the link even if an individual myosin-1A lets go. The link appears to be specific as other raft proteins—which probably come and go between different rafts—are not affected by the dominant-negative constructs. Tyska is now determining whether the myosin-1A–SI link is regulated by signaling proteins or bacterial toxins. ▪