Structural Organization of the Augmin Complex Involved in Branching Microtubule Nucleation

Abstract

Microtubules are critical cellular structures involved in many processes of the eukaryotic cell, especially in cell division. They are built and organized by nucleation, polymerization and transport events involving a complex network of proteins in the spindle to mediate chromosome segregation. One particular microtubule nucleation pathway that has been revealed to play an important role in mitotic spindle assembly is microtubule-dependent microtubule nucleation, also called branching microtubule nucleation. This pathway has been shown to be dependent upon several molecular factors, one of which is the augmin protein complex. Although it is known that augmin plays an essential role in branching microtubule nucleation, its subunit interactions and structure are not known, thus a precise mechanistic role for augmin has yet to be defined. Here, we establish augmin subunit interactions as well as successfully purify stable sub- complexes that form within augmin, which include two tetrameric complexes that bind to form the full-length structure. Our findings lead to a partial assembly model that reveals a more refined connectivity map for the whole augmin than previously proposed.