--Keqin Chen, John Hunsperger, and Irwin Rubenstein

Maize endosperm protein bodies contain two types of prolamins: the zeins (about 85% of the total) and the zein associated proteins (ZAPs). Three genes encoding for ZAPs have been isolated and characterized: the genes for 10kD, 15kD, and 27kD proteins.

A cDNA clone encoding a new ZAP has been isolated from a cDNA library made from W22 protein body mRNAs (22 DAP). A clone (Z19) which weakly hybridized to a Z36 probe (coding for the 27kD ZAP) was isolated. Northern blots indicated a 1kb mRNA. Hybridization release - in vitro translation experiments produced a protein product with a relative molecular weight of 26kD. The sequence of the cDNA clone indicated a 785bp insert with a 549bp open reading frame. The deduced amino acid sequence consists of 183 amino acid residues with an absolute molecular weight of 19,560kD. The DNA sequence is high in GC (about 67%).

The Z19 amino acid sequence in some ways resembles the Z36 sequence. The exact length of its signal peptide is unknown, but the amino acid sequence is similar to the Z36 signal peptide (Fig. 1). The basic residue near the N-terminal of the Z19 signal peptide, however, is Lys instead of Arg as in Z36. A hexapeptide (Pro-Pro-Pro-Phe-Tyr-X) near the N-terminal of the mature protein is repeated twice in Z19. In Z36, the hexapeptide (Pro-Pro-Pro-Val-His-Leu) is repeated 8 times with only one replacement amino acid residue in the seventh repeat. The N- terminal and the C-terminal amino acid sequences of the mature Z19 and Z36 proteins are conserved (Fig. 2, 3). An interesting difference is that the last 2 amino acid residues at the C- terminus of Z19 are identical to those of the 15kd ZAP. Southern genomic blots indicate only one copy of the Z19 gene in the inbred W22.
 

CZ15A3	M-K-M-V-I-V-L-L-L-W-L-A-L-S-A-A-S-A-S-A
Z19	M-K---V-L-I-V-A-L-A-L-L-A-L-A-A-S-A-A(S)(S)?
Z36	M-R---V-L-L-V-A-L-A-L-L-A-L-A-A-S-A-T-S

Fig. 1. Comparison of the signal peptide sequences.
 

CZ15A3	S-Y-R-T-N-P-C-G-V-S-A-A-I-P-P-Y-Y
Z19	Q-Q-P-G-P-C-P-C-N-A-A-A-G-G-V-Y-Y
Z36	Q-Q-P-T-P-C-P-Y---A-A-A-G-G-V-P-H

Fig. 2. Comparison of the C-terminal region peptide sequences.
 

CZ15A3	M-Q-M-P-C-P-C-A-G-L-Q
Z19	(S)(S)?T-S-G-G-C-G-C-Q-T
Z36	T-H-T-S-G-G-C-G-C-Q-P

Fig. 3. Comparison of the N-terminal region peptide sequences.
 

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