NAK kinases, also known as numb-associated kinase. During asymmetric cell division, the membrane-associated Numb protein are located at a crescent in the mitotic progenitor cell and is mainly isolated in one of two daughter cells. We identified a putative serine/threonine kinase, Numb-related kinase (Nak), that physically interacts with Numb's phosphotyrosine-binding (PTB) domain, Shc, and PTB for insulin receptor substrates The domain binds to an NPXY motif that is not present in the Nak region that interacts with the Numb PTB domain. We found that the Numb PTB domain, but not the Shc PTB domain, interacts with Nak through peptides of type II amino acids, which implies a novel and specific protein-protein interaction. The overexpression of Nak in sensory organs leads to normal The two daughters of the asymmetric cell division adopt the same cell fate, this transition is similar to the loss of numbness phenotype, and is contrary to the cell fate transformation caused by Numb overexpression, the frequency of cell fate transformation is sensitive to the numb gene dose , Which is expected from physical interactions.

Numb Protein

Protein numb homolog is a human protein encoded by the NUMB gene. The protein encoded by this gene plays a role in determining cell fate during development. The encoded protein that MDM2 induces its degradation in a proteasome-dependent manner is a membrane-bound protein that has been shown to be associated with EPS15, LNX1, and NOTCH1. The gene has found four transcript variants encoding different isoforms. The Numb protein is encoded by the NUMB gene, and its mechanism appears to be evolutionarily conserved. Numb has been extensively studied in invertebrates and mammals, although its function is best understood in fruit flies. Numb plays a vital role in asymmetric cell division during development, thereby making the cell fate of the central nervous system and the peripheral nervous system different. During neurogenesis, Numb is localized to one side of the mother cell and is thus selectively distributed in one daughter cell. This asymmetric division gives Numb-containing daughter cells a different fate than other daughter cells.

Figure 1. Protein structure of Numb protein.

Role in cell migration

Neural precursors are first produced in the hyperplasia area and then migrate to a targeted location, where they undergo maturation and become functional neurons. Studies in Drosophila first showed that Numb plays a role in cell migration when mutants show defects in glial cell migration along axons. Since then, a mechanism has been discovered through which Numb binds chemotactic signal receptors to form scaffolds that recruit atypical PKC (aPKC) to the receptor complex. Once activated, aPKC phosphorylates Numb, which promotes a forward feedforward response, thereby enhancing the binding of Numb chemotactic receptors and subsequently forming endosome complexes. Endocytosis supports the relocation of chemokine receptors to the front of the cell to promote receptor-mediated directed migration in response to receptor activation.

Inhibition of Notch signaling via ubiquitination pathway

Numb exerts its functional role in cell fate determination by antagonizing Notch signaling activity. The molecular mechanism behind this relationship seems to depend on the ubiquitination of the membrane bound Notch1 receptor. To prove this, Numb's ability to generalize Notch1 was directly related to its functional inhibition of Notch1 signaling activity. The ubiquitination pathway directs protein recovery by directly labeling specific proteins for proteasome degradation. Through a multi-step process, free ubiquitin is first linked to activating enzyme (E1) and then transferred to coupling enzyme (E2), which binds to ligase (E3), which acts as an adaptor to selectively bind ubiquitin Transfer to specific protein substrates. Numb expression was found to selectively label the membrane Notch1 receptor for ubiquitination through the interaction of its phosphotyrosine binding domain with E3 ubiquitin ligase Itch. Numb and Itch work together to promote ubiquitination of full-length membrane-linked Notch receptors before activation. However, Numb seems to only promote the degradation of NICD cleavage products after receptor activation, targeting it for proteasome degradation and preventing its transfer to the nucleus.

Reference

1. Dho SE; et al. Characterization of four mammalian numb protein isoforms. Identification of cytoplasmic and membrane-associated variants of the phosphotyrosine binding domain. J. Biol. Chem. 1999, 274 (46): 33097-104.