Filopodia are dynamic, actin-rich finger-like structures that protrude from the 
cell membrane and play important roles in cell migration and neurite initiation 
and outgrowth. The insulin receptor substrate protein of 53 kDa (IRSp53) and the 
mammalian Diaphanous members of the formin family of proteins (mDia) are two key 
players in the formation of filopodia and neurites. IRSp53 is an adaptor protein 
that acts at the membrane:actin interface, coupling membrane deformation with 
F-actin polymerization. mDia formin proteins, instead, can nucleate and 
polymerize linear actin filaments. Emerging genetic and biochemical evidence 
indicate that there are multiple and independent pathways leading to filopodium 
and neurite formation, but the precise molecular components of these pathways 
remain ill-defined. We recently identified the PDZ domain-containing protein 
LIN7 as a novel regulator of IRSp53. We further showed that the association 
between these two proteins is required to promote the formation of filopodia and 
neurites independently from mDia formin proteins, highlighting novel mechanisms 
of filopodia and neurite formation.