A disease of actin transport?

Spinal muscular atrophy (SMA), a motoneuron disease that results in paralysis and death usually before age 3, is caused by loss of the SMN1 gene. But what does the established splicing function of SMN1 have to do with motoneurons? Perhaps very little, say Rossoll et al., who on page 801 show that SMN1 is part of a complex that drags β-actin mRNA out to growth cones so that axons can grow and possibly function properly.The authors first looked at motoneuron survival in vitro. Survival of cells from a mouse SMA model was unimpaired, but axon growth, growth cone size, and axonal accumulation of actin was reduced. The SMN protein has been shown to associate with the RNA-binding protein hRNP R, and Rossoll et al. found that hRNP R associates, in turn, with β-actin mRNA. The axonal concentration of both...