While many genes are known to be required for spindle pole body duplication, the best studied are perhaps the conserved family of SUN-domain proteins. The SUN-protein homolog in yeast is Mps3, an integral membrane component of the spindle pole body required for early steps in the duplication process.
"Cells with little or no functional Mps3 do not divide, and have only one spindle pole body and one half of the mitotic spindle," explains Jaspersen. "We were interested in how a spindle pole body gets inserted into the nuclear envelope, what modifications of this double lipid bi-layer membrane have to occur to facilitate insertion, and what is Mps3's role in all of this?"
To better understand the function of Mps3 in spindle pole body duplication, Jaspersen's team, led by co-first authors Jennifer Friederichs and Suman Ghosh, Ph.D., mutated specific regions of the Mps3 gene and then expressed the mutated genes in yeast. For most of the mutants, mitosis seemed normal. That wasn't the case, however, with one particular novel mutant, MPS3-G186K, which has a small, "point" mutation in the so-called P-loop region.
The researchers next used high-resolution electron microscopy and various markers, including ones that can distinguish uninserted and inserted spindle pole bodies. What they saw was that, although their DNA had been duplicated, cells expressing this particular Mps3 mutant had multiple duplication defects, including blocking insertion of the spindle pole body into the nuclear envelope.
What was most striking, however, was that nearly every cell examined had nuclear membranes that were, essentially, overgrownwith two to eight layers of nuclear envelope, and multiple lobes and extensions -- instead of a simple spherical structure. Importantly, the effect seemed specific in that the other membrane-based organelles
|Contact: Gina Kirchweger|
Stowers Institute for Medical Research