The septation apparatus, an autonomous system in budding yeast.

Actomyosin ring contraction and chitin primary septum deposition are interdependent processes in cell division of budding yeast. By fusing Myo1p, as representative of the contractile ring, and Chs2p for the primary septum, to different fluorescent proteins we show herein that the two processes proceed essentially at the same location and simultaneously. Chs2p differs from Myo1p in that it reflects the changes in shape of the plasma membrane to which it is attached and in that it is packed after its action into visible endocytic vesicles for its disposal. To ascertain whether this highly coordinated system could function independently of other cell cycle events, we reexamined the septum-like structures made by the septin mutant cdc3 at various sites on the cell cortex at the nonpermissive temperature. With the fluorescent fusion proteins mentioned above, we observed that in cdc3 at 37 degrees C both Myo1p and Chs2p colocalize at different spots of the cell cortex. A contraction of the Myo1p patch could also be detected, as well as that of a Chs2p patch, with subsequent appearance of vesicles. Furthermore, the septin Cdc12p, fused with yellow or cyan fluorescent protein, also colocalized with Myo1p and Chs2p at the aberrant locations. The formation of delocalized septa did not require nuclear division. We conclude that the septation apparatus, composed of septins, contractile ring, and the chitin synthase II system, can function at ectopic locations autonomously and independently of cell division, and that it can recruit the other elements necessary for the formation of secondary septa.