KIF3C (green) motors apically along microtubules (red).

To maintain distinct populations of proteins at their apical and basolateral membranes, polarized cells package and sort membrane proteins into vesicles, which then dock specifically at one membrane or the other. But is there also a selective transport system to carry the vesicles to their destinations, and, if so, how does it work? Noda et al. report on page 77 that a new kinesin family member, KIFC3, can act as a microtubule motor associated with apically targeted transport vesicles, apparently independently of cytoplasmic dynein–dynactin-dependent transport.

As microtubules in polarized cells extend their minus ends apically, the authors cloned and characterized KIFC3, a putative minus-end–directed microtubule motorprotein uncovered in an earlier search for novel kinesins. KIFC3 is concentrated on membrane organelles near the apical plasma membrane of polarized epithelial cells, colocalizing with annexin XIIIb, a membrane protein found in apically targeted transport vesicles.

Overexpressing a dominant-negative form of KIFC3 partially inhibits apically targeted transport. Interestingly, the KIFC3-associated vesicles lack cytoplasmic dynein, and electron microscopy confirms that the two motor proteins do not colocalize, suggesting that they drive independent transport pathways. Because KIFC3 expression levels vary significantly between tissues, the authors propose that cytoplasmic dynein may drive constitutive apical transport, which could be supplemented by KIFC3-mediated transport in some cell types. ▪