Seybold et al. describe how three proteins interact to form a structure that initiates the duplication of spindle pole bodies (SPBs), the yeast equivalent of centrosomes.
Budding yeast SPBs are embedded in the nuclear envelope and must be duplicated once per cell cycle. A structure called the half bridge juts out from the SPB on both the nuclear and cytoplasmic sides of the nuclear envelope. The cytoplasmic part of this structure contains ∼60 Sfi1 molecules, aligned in parallel with their N termini embedded in the SPB’s central plaque. In G1, the distal C-terminal domains of these Sfi1 molecules bind to the C termini of additional Sfi1 molecules, extending the half bridge into a bridge. A new SPB then assembles around the Sfi1 N-terminal domains exposed at the far end of this structure.
Two other proteins, Kar1 and Cdc31, localize to the cytoplasmic region of the half bridge/bridge, but their roles in SPB duplication remain unclear.
Seybold et al. used super-resolution microscopy to show that Kar1, an integral nuclear membrane protein, localizes to the center of the bridge. Accordingly, Kar1 bound to the C-terminal region of Sfi1 that forms this part of the structure. In the absence of Kar1, the bridge arched away from the nuclear envelope as if it was no longer anchored to the outer nuclear membrane. Artificially cross-linking Sfi1 to Kar1’s transmembrane domain flattened out the bridge and restored cell viability.
Cdc31, a homologue of the centrosomal protein centrin, binds to both Kar1 and Sfi1. Seybold et al. found that the protein stabilizes the half bridge/bridge by cross-linking neighboring Sfi1 molecules. Indeed, says senior author Elmar Schiebel, all three proteins stabilize each other’s presence in the structure in order to promote SPB duplication.
Text by Ben Short