Actin patches (green) around endocytosed material (red) ride actin cables out of the bud (left to right).

On page 519, Huckaba et al. show that endosomes ride an actin-based conveyor belt from bud to mother cell in yeast. This passive mode of transport may replenish membrane fusion components needed for polarized secretion.

Lots of polarized secretion is needed to build a new bud. Secretory vesicles are brought into the bud via active transport along actin cables. These cables are actin bundles that are lengthened by the addition of new actin at the bud site, which thereby produces a rearward flow of actin into the mother cell.

This rearward flow is now shown to carry another actin structure, called an actin patch, into the mother cell. Using an endocytosed label, the authors demonstrate that patches are the actin-rich coats of endosomes. After budding off the membrane at secretion sites, where actin patches are enriched, many endosomes latched onto actin cables and rode them into the mother cell at the rate of the cables' rearward actin flow. Retrograde actin flow is common among higher eukaryotes, where it may also drive vesicle movement.

The patches disassembled when the vesicles met, and presumably fused with, endosomal sorting compartments, from which SNAREs and other secretion proteins can be recycled. Some endosomes avoided cables and remained in the bud. Perhaps the ability to latch onto cables is only present in vesicles containing certain cargoes.