Figure 2.3
Notch, its domains, and some of its binding partners.
Part of a cell is shown (gray = cytoplasm; microvilli at top) with two Notch (middle) and one Numb molecule (lower right) drawn as bars (cf. scale at upper left). Domains (variously shaded or hatched) are mapped. Other proteins (black rectangles) are not to scale. Partners are linked by hooks (cf. key), and binding sites are delimited by dashed ovals or half ovals.

Despite their regularity, the various EGF repeats play different roles [459, 4601, 4823], and no two are identical [2182, 2542]. Delta and Serrate both bind EGF repeats 11 and 12 [459, 985, 3544], though they also rely on repeats 24-26 (not shown) [2419], and binding alone may not suffice for activation [1943, 1944]. Fringe (cf. Ch. 6) binds repeats 24-29 [990] and at the LNG domain [2096]. Scabrous requires repeats 19-26 for its association with N (not shown) [3456], though any binding must be indirect [4601]. The famous allele split (whence the link to the E(spl)-C was deduced [3028]) is due to a missense mutation (Ile-to-Thr) in repeat 14 [1747, 2182], and the widely used heat-sensitive allele N^ts1 has a missense mutation (Gly-to-Asp) in repeat 32 (a.a. 1272) [4779].

Notch is cleaved at three sites (zigzag lines) [491, 2995, 4809]. Cleavage at site #1 occurs during maturation in the trans-Golgi along the secretory pathway [368, 2299], and the fragments stay together -- possibly by disulphide bridges at C1693 and C1696 [3153] (but see [446, 578]). Cleavage at site #2 occurs after ligand binding [491, 2995] -- releasing an ectodomain that is 'swallowed' by the ligand-bearing cell [2263, 3271] (cf. the reverse with Boss [601, 2318, 4211]). The smaller size of the extracellular vestige (now <300 a.a.) stimulates a Presenilin-dependent protease to make the next (final?) cut [3533, 4156, 4582] . Cleavage at site #3 occurs in the transmembrane domain (a.a. 1746-1765) between M1762 and V1763 [2995, 3805] -- releasing an intracellular fragment (N-intra) that goes to the nucleus and turns ON target genes as a co-activator with Su(H) [4155] (see text for evidence). For review see [2994].

Numb -- when present -- apparently tethers N-intra to the membrane (cf. Fig. 2.2). Half of Numb's PTB domain binds Notch at RAM23 -- where Su(H) also binds [1307, 1651, 2747, 4244] -- and at a less crucial C-terminal site [1139, 1307, 1651]. Deltex may displace Su(H) from Notch when Delta is absent [112, 1448, 2746, 3022], though binding sites for Deltex and Su(H) don't overlap. (Deltex switches cell fates when overexpressed but has no LOF effect on bristles [2746].) Notch is thought to dimerize via cysteines (C1693 and C1696?) [2209, 2542] but may also do so via its ankyrin repeats [2747] or opa motif (link not shown).

Binding of Dishevelled (Dsh) to the C-terminal tail short-circuits the Notch and Wingless signaling pathways. Notch's other binding partners include Wingess itself (see App. 3 for other modulators, [not available on-line]). Sites of phosphorylation [368, 2209, 2211, 4582] and glycosylation [2070, 4611] are not shown. Fringe is presumed to glycosylate Notch at O-linked-fucose sites in EGF repeats 3, 20, 24, 26, and 31 [2904]. Adapted from [368, 446, 2210, 2542, 4611]. See [4602, 4603] for apparently heretical modes of Notch signaling. See also App. 7.