Chant J, Mischke M, Mitchell E, Herskowitz I, Pringle J R
Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
J Cell Biol. 1995 May;129(3):767-78. doi: 10.1083/jcb.129.3.767.
Yeast cells can select bud sites in either of two distinct spatial patterns. a cells and alpha cells typically bud in an axial pattern, in which both mother and daughter cells form new buds adjacent to the preceding division site. In contrast, a/alpha cells typically bud in a bipolar pattern, in which new buds can form at either pole of the cell. The BUD3 gene is specifically required for the axial pattern of budding: mutations of BUD3 (including a deletion) affect the axial pattern but not the bipolar pattern. The sequence of BUD3 predicts a product (Bud3p) of 1635 amino acids with no strong or instructive similarities to previously known proteins. However, immunofluorescence localization of Bud3p has revealed that it assembles in an apparent double ring encircling the mother-bud neck shortly after the mitotic spindle forms. The Bud3p structure at the neck persists until cytokinesis, when it splits to yield a single ring of Bud3p marking the division site on each of the two progeny cells. These single rings remain for much of the ensuing unbudded phase and then disassemble. The Bud3p rings are indistinguishable from those of the neck filament-associated proteins (Cdc3p, Cdc10p, Cdc11p, and Cdc12p), except that the latter proteins assemble before bud emergence and remain in place for the duration of the cell cycle. Upon shift of a temperature-sensitive cdc12 mutant to restrictive temperature, localization of both Bud3p and the neck filament-associated proteins is rapidly lost. In addition, a haploid cdc11 mutant loses its axial-budding pattern upon shift to restrictive temperature. Taken together, the data suggest that Bud3p and the neck filaments are linked in a cycle in which each controls the position of the other's assembly: Bud3p assembles onto the neck filaments in one cell cycle to mark the site for axial budding (including assembly of the new ring of neck filaments) in the next cell cycle. As the expression and localization of Bud3p are similar in a, alpha, and a/alpha cells, additional regulation must exist such that Bud3p restricts the position of bud formation in a and alpha cells but not in a/alpha cells.
酵母细胞可以通过两种不同的空间模式选择出芽位点。a细胞和α细胞通常以轴向模式出芽，即母细胞和子细胞都在先前分裂位点附近形成新的芽。相比之下，a/α细胞通常以双极模式出芽，即新的芽可以在细胞的任一极形成。BUD3基因是轴向出芽模式所特需的：BUD3的突变（包括缺失）会影响轴向模式，但不影响双极模式。BUD3的序列预测其产物（Bud3p）有1635个氨基酸，与先前已知的蛋白质没有明显的或指导性的相似性。然而，Bud3p的免疫荧光定位显示，在有丝分裂纺锤体形成后不久，它会在环绕母芽颈部的明显双环中组装。颈部的Bud3p结构一直持续到胞质分裂，此时它分裂形成单个Bud3p环，标记两个子代细胞中每个细胞的分裂位点。这些单环在随后的大部分未出芽阶段都存在，然后解体。Bud3p环与颈部细丝相关蛋白（Cdc3p、Cdc10p、Cdc11p和Cdc12p）的环没有区别，只是后者的蛋白质在芽出现之前组装，并在细胞周期持续期间保持在原位。当温度敏感的cdc12突变体转移到限制温度时，Bud3p和颈部细丝相关蛋白的定位会迅速丧失。此外，单倍体cdc11突变体在转移到限制温度时会失去其轴向出芽模式。综上所述，数据表明Bud3p和颈部细丝在一个循环中相互关联，其中每个都控制着另一个组装的位置：Bud3p在一个细胞周期中组装到颈部细丝上，以标记下一个细胞周期中轴向出芽的位点（包括新的颈部细丝环的组装）。由于Bud3p在a、α和a/α细胞中的表达和定位相似，所以必须存在额外的调控，使得Bud3p限制a和α细胞中芽形成的位置，但不限制a/α细胞中芽形成的位置。
