Neck compartmentalization as the molecular basis for the different endocytic behaviour of Chs3 during budding or hyperpolarized growth in yeast cells.

Yeast cells normally grow by budding, but under certain specific conditions they are also able to grow in hyperpolarized forms reminiscent of hyphal growth. During vegetative growth, the synthesis of the septum that physically separates yeast cells during cytokinesis depends on the correct assembly of the septin ring. Septins and actin patches are assembled at the neck, forming two concentric rings where the actin patch ring occupies the external-most part. This specific positioning defines a plasma membrane region at the neck from which other lateral membrane compartments are excluded. In this scenario, correct assembly of the chitin ring is dependent on the anchoring of Chs3 to the septin ring through Chs4. The anchoring of Chs3 to septins through Chs4 prevents the arrival of this protein at endocytic sites, thus reducing the endocytosis of Chs3. This allows an equilibrium to be set up between the antero- and retrograde transport of Chs3, facilitating the synthesis of the chitin ring at the neck. In contrast, hyperpolarized growth is characterized by a reduced endocytic turnover of Chs3, which in turn lead to the accumulation of Chs3 at the plasma membrane and a concomitant increase in chitin synthesis.