Numb Is an Endocytic Protein

Abstract

Abstract. Numb is a protein that in Drosophila determines cell fate as a result of its asymmetric partitioning at mitosis. The function of Numb has been linked to its ability to bind and to biologically antagonize Notch, a membrane receptor that also specifies cell fate. The biochemical mechanisms underlying the action of Numb, however, are still largely unknown. The wide pattern of expression of Numb suggests a general function in cellular homeostasis that could be additional to, or part of, its action in fate determination. Such a function could be endocytosis, as suggested by the interaction of Numb with Eps15, a component of the endocytic machinery.Here, we demonstrate that Numb is an endocytic protein. We found that Numb localizes to endocytic organelles and is cotrafficked with internalizing receptors. Moreover, it associates with the appendage domain of ␣ adaptin, a subunit of AP2, a major component of clathrin-coated pits. Finally, fragments of Numb act as dominant negatives on both constitutive and ligandregulated receptor-mediated internalization, suggesting a general role for Numb in the endocytic process.

Numb regulates vesicular docking for homotypic fusion of early endosomes via membrane recruitment of Mon1b

“…Numb 66 and Numb 72 are attached on the plasma membrane and have an 11-aa insertion in the PTB domain. In contrast, Numb 65, Numb 71 and Numblike lack the 11-aa insertion in their PTB domain, and are mostly localized in the cytosol [15,17]. These observations suggest that the presence of the 11-aa fragment within the PTB domain affects cell membrane localization of Numb proteins.…”

“…Numb carries out these functions via regulating endocytic trafficking of several key molecules, such as Notch, sanpodo, cadherins and integrins [4,7,9,14,17,18,23,24,26,51,52]. Although in all these contexts Numb functions as an endocytic adaptor protein, the exact role of Numb in endocytic trafficking remains an open question.…”

“…Numb localizes to endocytic organelles and participates in both clathrin-dependent and clathrin-independent endocytic trafficking of a number of key molecules such as Notch, EGFR, transferrin, integrin, N-cadherin, E-cadherin and L1 (a neuronal cell adhesion molecule) [9,[17][18][19][20][21]. Genetic evidence shows that Numb contributes to cell fate determination by antagonizing Notch activity in one of the two daughter cells after asymmetric cell division [2,3,22].…”

“…Presumably, Numb 65, Numb 66 and Numblike promote npg cell differentiation, whereas Numb 71 and Numb 72 promote cell proliferation [15,16]. Different Numb proteins are also distinctly localized to different subcellular compartments: Numb 65, Numb 71 and Numblike are mostly localized in the cytosol, whereas Numb 66 and Numb 72 are mostly localized to the plasma membrane [17], suggesting that different Numb isoforms have distinct roles in different compartments.…”

Novel functions of endocytic player clathrin in mitosis

“…Moreover, other studies also suggested the potential cooperation between mitotic kinases and clathrin. The endocytic protein Numb, which directly associates with clathrin-associated adaptor complex AP-2 [34], is phosphorylated by AAK1 to promote clathrincoated pits maturation [35]; while in asymmetric cell [36][37][38]. With no doubt, in the cell, the crosstalk among clathrin, Ran-GTP and mitotic kinases could be much more complicated than expected and more regulators need to be found.…”