Probes for small transfer cell-specific proteins
--Willmott, RL; Hueros, G; Varotto, S and Thompson, RD
A cDNA clone was isolated from a cDNA bank constructed from 10-days after pollination (DAP) endosperm mRNA and has been characterised in detail (Hueros et al., Plant Cell 7:747-757, 1995). This was the first reported gene to be exclusively expressed in basal region of the endosperm. This area is highly specialised to facilitate uptake of solutes during grain development. Due to the location of the expression of the gene it was referred to as BET1 (for basal endosperm transfer layer). Subsequently a bet1 (glycinebetaine 1) locus was found to exist in MaizeDB (#40554), therefore to minimize confusion we suggest the transfer cell-specific genes be referred to as betl.
So far two transfer cell-specific cDNAs have been isolated (betl1 and betl2, ID #105963 and 105964 in the MaizeDB respectively). betl1 belongs to a small multigene family of which four different members have been characterised, and two copies of betl2 are present in the maize genome. Both genes are strongly expressed between 9 to 20 DAP, betl2 being more highly expressed then betl1. The proteins encoded by betl1 and betl2 share some common features: the deduced amino acid sequences comprise small proteins with calculated Mr of 7 kD. The sequences start with a hydrophobic region characteristic of a signal peptide and the encoded proteins are cysteine rich. The betl1 polypeptide contains one copy of the extensin motif, SPPPP and is found in cell wall fractions. The function of betl2 remains to be eludicated, however the protein has two interesting features, a potential glycosylation site and the possibility of numerous disulphide bond formations. Neither betl1 nor betl2 share obvious similarities with sequences in current databases.
Genomic clones corresponding to the betl1 cDNA have been isolated and characterised. Interestingly two clones were derived from a distinct but closely related locus, provisionally termed betl3. The betl3 coding sequence displays 90% similarity to betl1. From the predicted amino acid sequence it is evident that the proteins contain no extensin motif. Preliminary evidence indicates that betl3 expression may not be limited to the transfer cell layer, which may point to a different role for this protein.
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