Worm Breeder's Gazette 11(2): 45

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

EGF-like Repeats in C. elegans Gene Products

John Yochem and Iva Greenwald

The lin-12 and glp-1 gene products are composed in part of repeats 
of an amino acid sequence motif that resembles epidermal growth factor.
The lin-12 product contains 13 copies of the motif, and the glp-1 
product contains 13 copies (Greenwald, 1985; Yochem et al., 1988; 
Yochem and Greenwald, 1989; Austin and Kimble, 1989; Priess and Fire, 
personal communication).  We have attempted to isolate additional 
genes encoding proteins with EGF-like repeats.  We reasoned that the 
products of such genes might be critical for development.  For example,
like lin-12 and glp-1, these products might be integral membrane 
proteins that mediate cell-cell interactions.  Another possibility is 
that such products might be components of the extracellular matrix.  
By low-stringency hybridization with probes derived from both lin-12 
and glp-1, we have been able to isolate several genomic clones that 
encode products with amino acid sequences that bear at least some 
resemblance to the EGF motif.
The product predicted for one clone shares with EGF a characteristic 
array of six cysteine residues, although the other amino acids 
conserved in EGF-like repeats are absent.  Clusters of tandem repeats 
of the array are separated by proline-rich stretches of amino acids.  
There are at least 23 copies of the array, and the product appears to 
lack a membrane-spanning domain.
Based on partial DNA sequences, a second clone encodes a protein 
with tandem repeats of a motif based on 8 cysteine residues that 
closely resembles the EGF-like regions of the mammalian B1 and B2 
chains of the extracellular matrix protein laminin.  The predicted 
amino acid sequence is distinct from that of the B2 chain-like product 
of the unc-6 gene (Ishii et al., CSH 1989), and the clone has been 
positioned to a different chromosome than unc-6 (Sulston and Coulson, 
personal communication).  The product of this clone may be the A or B1 
chain of laminin or a novel molecule related in structure.
A third clone encodes a product with a remarkable resemblance to the 
membrane bound LDL receptor of mammals and, to a lesser extent, the 
membrane-bound EGF precursor of mammals.  The product has all of the 
major features of the LDL receptor including a ligand binding region 
composed of multiple copies of an acidic, non-EGF-like cysteine repeat.
As in the LDL receptor, this region is followed by two tandem copies 
of the EGF-like motif, a stretch of about 275 amino acids, and then a 
single copy of the EGF motif.  A similar region of two EGF-like 
repeats, a stretch of 275 amino acids, and a single EGF-like copy is 
also present in the EGF precursor, but the EGF precursor contains 
additional EGF-like repeats (one of which is EGF in precursor form) 
and lacks the LDL ligand binding region.  The C.  elegans product is 
expected to be much larger than the mammalian LDL receptor because the 
overall structure discussed above is repeated at least four times in 
tandem with some variability in the number of amino acids between 
repeat units.  Only the COOH-terminal most repeat unit is followed by 
a potential membrane spanning domain.  One obvious possibility is that 
this protein is a direct counterpart of the mammalian LDL receptor and 
mediates a general or cell-specific endocytosis of cholesterol in the 
worm.  Alternatively, it could function in the endocytosis of yolk 
proteins that are synthesized in the gut and accumulate in oocytes.  
We have also considered the possibility that it is a membrane-bound 
signalling molecule or precursor of a secreted signal.  The product 
could, of course, have a combination of functions.  The gene has been 
placed to the right of unc-13 on LGI (Sulston and Coulson, personal 
communication).