An early nodulin-like protein accumulates in the sieve element plasma membrane of Arabidopsis

Membrane proteins within the sieve element-companion cell complex have essential roles in the physiological functioning

of the phloem. The monoclonal antibody line RS6, selected from hybridomas raised against sieve elements isolated from

California shield leaf (Streptanthus tortuosus; Brassicaceae) tissue cultures, recognizes an antigen in the Arabidopsis (Arabidopsis

thaliana) ecotype Columbia that is associated specifically with the plasma membrane of sieve elements, but not companion

cells, and accumulates at the earliest stages of sieve element differentiation. The identity of the RS6 antigen was revealed by

reverse transcription-PCR of Arabidopsis leaf RNA using degenerate primers to be an early nodulin (ENOD)-like protein that

is encoded by the expressed gene At3g20570. Arabidopsis ENOD-like proteins are encoded by a multigene family composed of

several types of structurally related phytocyanins that have a similar overall domain structure of an amino-terminal signal

peptide, plastocyanin-like copper-binding domain, proline/serine-rich domain, and carboxy-terminal hydrophobic domain.

The amino- and carboxy-terminal domains of the 21.5-kD sieve element-specific ENOD are posttranslationally cleaved from

the precursor protein, resulting in a mature peptide of approximately 15 kD that is attached to the sieve element plasma

membrane via a carboxy-terminal glycosylphosphatidylinositol membrane anchor. Many of the Arabidopsis ENOD-like proteins

accumulate in gametophytic tissues, whereas in both floral and vegetative tissues, the sieve element-specific ENOD is

expressed only within the phloem. Members of the ENOD subfamily of the cupredoxin superfamily do not appear to bind

copper and have unknown functions. Phenotypic analysis of homozygous T-DNA insertion mutants for the gene At3g20570

shows minimal alteration in vegetative growth but a significant reduction in the overall reproductive potential.