Figure 2.

Models for cell cycle progression in yeast, pseudohyphal and hyphal cells.

A. Yeast cells traverse START by forming a septin ring (orange), initiating bud emergence directed by a polarisome (red crescent) and duplicating the spindle pole body (yellow). Growth becomes less polarized as sites of growth (red) become distributed around the bud. In G2 phase the nucleus (blue) moves to the neck assisted by astral microtubule (green) sliding along the cortex and, at anaphase, divides across the neck. At telophase, the spindle disassembles, growth is focused at the neck, the septin ring splits into two and then each ring disappears prior to appearance of the next ring in G1. Right inset, polarisome protein Mlc1p-YFP localizes to the tip during early bud growth. Left inset, delocalized actin (red) patches reflect isotropic growth.

B. Pseudohyphal cells have similar features to yeast cells with a few exceptions: the polarisome persists for longer and cells spend more time in G2 phase, becoming similar in size to mother cells; cells do not separate following cytokinesis. As in yeast cells, sites of growth are cell cycle dependent, leaving the tip and focusing at the bud neck prior to cytokinesis. Right inset, Mlc1p-GFP (green) appears at the tips of small and larger buds. Left inset, at cytokinesis, Mlc1p-GFP disappears from bud tip and localizes to the neck.

C. Upon induction of hyphal growth from a yeast cell, the Spitzenkörper (red circle) directs germ tube evagination, which persists throughout the cell cycle and initiates prior to START. A polarisome is also present at hyphal tips. Nuclei migrate to and divide across the presumptum, and the septin ring persists into the next cell cycle. Right inset, photomicrograph of Spitzenkörper protein Mlc1p-YFP (green); cell surface is labeled with Texas-red conjugated to Concanavalin A. Left inset, during cytokinesis Mlc1p-YFP remains at the growing tip and also appears at the septum.