The CPEB1 mutant (S174A, S180A) (CPEB1-AA) disrupts neurite outgrowth in vitro. (A-C) Model for how CPEB1-AA competitively interferes with the function of endogenous cytoplasmic polyadenylation element (CPE)-binding proteins. Normally, unidentified CPE-binding proteins regulate the translation or localization of CPE-containing mRNAs (A). The CEPB1-RNA binding mutant (C529A, C539A) (CPEB1-RBM), being unable to bind to CPE-containing RNAs, does not affect this situation and thus serves as a negative control (B). However, CPEB1-AA binds to the CPE, displacing native CPE-binding proteins, thus preventing CPE-mediated mRNA regulation (C). (D) Fewer retinal cells transfected with CPEB1-AA-GFP (AA) have neurites compared to cells transfected with CPEB1-RBM-GFP (RBM). (E) AA-expressing dissociated retinal neurons have shorter neurites than those expressing RBM. Neurite length was quantified by digitally tracing the longest neurite of each transfected neuron (see Materials and methods for details). (F, G) Dissociated retinal neurons transfected with RBM (F), or AA (G). Note the presence of CPEB1-GFP in the neurites. *p < 0.05; ***p < 0.001. Scale bars: 15 μm. Error bars represent standard error of the mean.