HOP-1, a Caenorhabditis elegans presenilin, appears to be functionally redundant with SEL-12 presenilin and to facilitate LIN-12 and GLP-1 signaling

Abstract

Mutant presenilins have been found to cause Alzheimer disease. Here, we describe the identification and characterization of HOP-1, a Caenorhabditis elegans presenilin that displays much more lower sequence identity with human presenilins than does the other C. elegans presenilin, SEL-12. Despite considerable divergence, HOP-1 appears to be a bona fide presenilin, because HOP-1 can rescue the egg-laying defect caused by mutations in sel-12 when hop-1 is expressed under the control of sel-12 regulatory sequences. HOP-1 also has the essential topological characteristics of the other presenilins. Reducing hop-1 activity in a sel-12 mutant background causes synthetic lethality and terminal phenotypes associated with reducing the function of the C. elegans lin-12 and glp-1 genes. These observations suggest that hop-1 is functionally redundant with sel-12 and underscore the intimate connection between presenilin activity and LIN-12/Notch activity inferred from genetic studies in C. elegans and mammals.

Figure 1

(A) Predicted protein sequence of HOP-1 and its alignment with the predicted protein sequences of C. elegans SEL-12, human PS1, and human PS2. The pileup program of the GCG–Wisconsin package was used to create this alignment (30). Amino acids that are identical between at least three of the four proteins are highlighted in black. The predicted transmembrane domains based on topological studies of SEL-12 (9) are overlined. Two additional features referred to in the Discussion are marked: a hydrophobic region found in SEL-12 (“the seventh hydrophobic region”), PS1, and PS2 that does not span the membrane is overlined with carets (^), and a hydrophobic region found in all four presenilins (the “tenth hydrophobic region” of SEL-12) that is not membrane spanning is overlined with dots. The sequence of SEL-12 is from ref. , with a minor correction as described in ref. . The PS1 sequence is from ref. , and the PS2 sequence is from ref. . (B) Amino acid identity among the C. elegans and human presenilins. The gap program of the GCG–Wisconsin package was used to calculate the percentage of amino acid identity (30).

Figure 2

Rescue of the sel-12 Egl and abnormal vulva phenotypes by HOP-1. The data are shown for transgenic lines generated by injecting a construct that places a sel-12(+) cDNA or a hop-1(+) cDNA under the control of sel-12 5′ flanking sequence at a concentration of 20 μg/ml. Each line in the histogram represents data for an independent transgenic line; the number of hermaphrodites scored is shown above each line. The transgene is indicated on the horizontal axis. The percentage of Egl⁺ hermaphrodites is indicated on the vertical axis. For pLSX control lines, Egl⁺ hermaphrodites were never seen. See Materials and Methods for details about generating and scoring transgenic lines.

Figure 3

HOP-1 topology. (A) Hydrophobicity plot of HOP-1, generated using the Kyte–Doolittle algorithm (31) (window size = 15). Transmembrane domains based on topological studies of SEL-12 (9) are numbered. The location of hydrophobic regions of SEL-12 that do not span the membrane are indicated by asterisks. Arrows indicate the position of LacZ and TM::LacZ fusions. (B) β-Galactosidase activity of HOP-1::LacZ and HOP-1::TM::LacZ fusion proteins. (C) Inferred topology of HOP-1, based on the data in B and sequence similarities with other presenilins.

Figure 4

Nomarski photomicrographs of sel-12(ar131) hermaphrodites. (A, C, and E) sel-12(ar131) hermaphrodites. (B, D, and F) Progeny of sel-12(ar131) hermaphrodites that were injected with hop-1 antisense RNA (see Materials and Methods). (A and B) The head region. The white arrow indicates the position of the posterior bulb; the black arrow indicates the position of the anterior bulb of the pharynx, which is often missing in progeny of hop-1 RNA-injected parents, as in glp-1 mutants (25, 26). (C and D) The tail region. The white arrow indicates the intestine; the black arrow indicates the rectum, which is often missing in progeny of hop-1 RNA-injected parents, as in lin-12 glp-1 double mutants (27). (E and F) The gonad of L4 hermaphrodites. In F , germ line proliferation is reduced, and the arrow indicates a region of the germ line undergoing premature spermatogenesis; these phenotypes are characteristic of glp-1 mutants (25, 26).