Ubiquitin stripping

he Eg5 kinesin helps build a bipolar spindle by sliding microtubules past each other at the spindle midzone. On page 473, Ganem and Compton show that bipolarity requires another kinesin, called Kif2a, that localizes to spindle poles. Kif2a is strongly expressed in neuronal cells and is a member of a class of kinesins that depolymerize micro-tubules instead of stepping along their surface. The authors find that Kif2a is also present in several other vertebrate cell types and that reduction in Kif2a activity resulted in monopolar spindles. Rescue experiments demonstrate that bipolar spindles fail to form following Kif2a inhibition because kineto-chores chew up microtubule plus ends and pull spindle poles together. Bipolar spindles were restored if micro-tubule plus ends were stabilized at kinetochores by inhibition of another depolymerizing kinesin, called MCAK. MCAK is needed to correct spindle attachment errors. Proper bipolar attachment generates tension that is thought to turn off the depolymerizing activity of MCAK. The work by Ganem and Compton suggests that Kif2a activity is also needed to create tension by lopping off tubulin subunits from the minus ends of spindle microtubules and thus initiating poleward microtubule flux. Slack microtubules resulting from the absence of Kif2a would thus lead to strong kinetochore activity and spindle collapse. ᭿ T Bipolar spindles form in cells lacking Kif2a if MCAK is also inhibited. o collagen XIX is hard to swallow, at least for mice lacking this extremely rare matrix protein. On page 591, Sumiyoshi et al. reveal that loss of collagen XIX disrupts muscle function in the murine esophagus. Collagen is the most abundant protein in the body, but there are at least 27 different types, some more common than others. Although collagen XIX is scarce, the authors find it is necessary for survival into adulthood. Mice that made no collagen XIX died within a few days after birth. These mice had an enlarged esophagus and signs of malnutrition. The distended esophagus was a result of impaired N Esophageal basement membranes (arrows) are thick and disorganized in collagen mutant mice (right). Ubiquitin stripping eceptors tagged for destruction can be rescued by enzymes that remove the tag, according to McCullough et al. (page 487). Thus, like kinases, ubiquitin ligases are opposed by R activities that undo their work. Activated internalized receptors are tagged with ubiquitin for sorting to lysosomes from endo-somes rather than recycling back to the plasma membrane. Until now, ubiquitin was thought …