Num1 versus NuMA: insights from two functionally homologous proteins.

In both animals and fungi, spindle positioning is dependent upon pulling forces generated by cortically anchored dynein. In animals, cortical anchoring is accomplished by a ternary complex containing the dynein-binding protein NuMA and its cortical attachment machinery. The same function is accomplished by Num1 in budding yeast. While not homologous in primary sequence, NuMA and Num1 appear to share striking similarities in their mechanism of function. Here, we discuss evidence supporting that Num1 in fungi is a functional homolog of NuMA due to their similarity in domain organization and role in the generation of cortical pulling forces.

Cortical dynein pulling mechanism is regulated by differentially targeted attachment molecule Num1.

Cortical dynein generates pulling forces via microtubule (MT) end capture-shrinkage and lateral MT sliding mechanisms. In Saccharomyces cerevisiae, the dynein attachment molecule Num1 interacts with endoplasmic reticulum (ER) and mitochondria to facilitate spindle positioning across the mother-bud neck, but direct evidence for how these cortical contacts regulate dynein-dependent pulling forces is lacking. We show that loss of Scs2/Scs22, ER tethering proteins, resulted in defective Num1 distribution and loss of dynein-dependent MT sliding, the hallmark of dynein function. Cells lacking Scs2/Scs22 performed spindle positioning via MT end capture-shrinkage mechanism, requiring dynein anchorage to an ER- and mitochondria-independent population of Num1, dynein motor activity, and CAP-Gly domain of dynactin Nip100/p150Glued subunit. Additionally, a CAAX-targeted Num1 rescued loss of lateral patches and MT sliding in the absence of Scs2/Scs22. These results reveal distinct populations of Num1 and underline the importance of their spatial distribution as a critical factor for regulating dynein pulling force.