Phosphatidylinositol 3-phosphate [PtdIns(3)P] regulates endocytic trafficking 
and the sorting of receptors through early endosomes, including the rapid 
recycling of transferrin (Tfn). However, the phosphoinositide phosphatase that 
selectively opposes this function is unknown. The myotubularins are a family of 
eight catalytically active and six inactive enzymes that hydrolyse PtdIns(3)P to 
form PtdIns. However, the role each myotubularin family member plays in 
regulating endosomal PtdIns(3)P and thereby endocytic trafficking is not well 
established. Here, we identify the myotubularin family member MTMR4, which 
localizes to early endosomes and also to Rab11- and Sec15-positive recycling 
endosomes. In cells with MTMR4 knockdown, or following expression of the 
catalytically inactive MTMR4, MTMR4(C407A), the number of PtdIns(3)P-decorated 
endosomes significantly increased. MTMR4 overexpression delayed the exit of Tfn 
from early endosomes and its recycling to the plasma membrane. By contrast, 
expression of MTMR4(C407A), which acts as a dominant-negative construct, 
significantly accelerated Tfn recycling. However, in MTMR4 knockdown cells Tfn 
recycling was unchanged, suggesting that other MTMs might also contribute to 
recycling. MTMR4 regulated the subcellular distribution of Rab11 and, in cells 
with RNAi-mediated knockdown of MTMR4, Rab11 was directed away from the 
pericentriolar recycling compartment. The subcellular distribution of VAMP3, a 
v-SNARE protein that resides in recycling endosomes and endosome-derived 
transport vesicles, was also regulated by MTMR4. Therefore, MTMR4 localizes at 
the interface of early and recycling endosomes to regulate trafficking through 
this pathway.