Catch the KIF5B train to the apical surface

Disparate Regulatory Mechanisms Control Fat3 and P75NTR Protein Transport through a Conserved Kif5-Interaction Domain

Directed transport delivers proteins to specific cellular locations and is one mechanism by which cells establish and maintain polarized cellular architectures. The atypical cadherin Fat3 directs the polarized extension of dendrites in retinal amacrine cells by influencing the distribution of cytoskeletal regulators during retinal development, however the mechanisms regulating the distribution of Fat3 remain unclear. We report a novel Kinesin/Kif5 Interaction domain (Kif5-ID) in Fat3 that facilitates Kif5B binding, and determines the distribution of Fat3 cytosolic domain constructs in neurons and MDCK cells. The Kif5-ID sequence is conserved in the neurotrophin receptor P75NTR, which also binds Kif5B, and Kif5-ID mutations similarly result in P75NTR mislocalization. Despite these similarities, Kif5B-mediated protein transport is differentially regulated by these two cargos. For Fat3, the Kif5-ID is regulated by alternative splicing, and the timecourse of splicing suggests that the distribution of Fat3 may switch between early and later stages of retinal development. In contrast, P75NTR binding to Kif5B is enhanced by tyrosine phosphorylation and thus has the potential to be dynamically regulated on a more rapid time scale.

Phosphoinositide signaling pathways: promising role as builders of epithelial cell polarity

Polarity is a prerequisite for proper development and function of epithelia in metazoa. The major feature of polarized epithelial cells is the presence of specialized domains with asymmetric distribution of macromolecular contents including proteins and lipids. The apical domain is involved in exchange with the organ lumen, and the basolateral membrane maintains contact with neighboring cells and the underlying extracellular matrix. The two domains are separated by tight junctions, which act as a diffusion barrier to prevent free mixing of domain-specific proteins and lipids. Extensive studies have shed light on the numerous protein families involved in cell polarization. However, many questions still remain regarding the molecular mechanisms of polarity regulation and in particular very little is known about the role of lipids in building polarity. In this chapter, essential determinants of epithelial polarity will be reviewed with a particular focus on metabolism and function of phosphoinositides.