A calcium-dependent protein kinase is associated with maize mesocotyl plasmodesmata

Plasmodesmata (Pd) are trans-wall membrane lined tunnels that regulate cell-to-cell cytoplasmic movement. It has been suggested that Pd conductivity may be regulated by a phosphorylation mechanism. In a I a maize (Zea mays L.) mesocotyl cell wall fraction, a Ca²⁺-dependent protein kinase (CDPK) is present that phosphorylates approximately 8 of 20 wall-associated proteins. The kinase is membrane-associated and is not extracted by EGTA, NaCl, up to 4 mol/L LiCl, Triton X-100, or Na₂CO₃ (pH 11), but is fully extracted with SDS or 8 mol/L LiCl. Two polypeptides in the cell wall fraction, with apparent molecular masses of 51 and 56 kD, cross-react with an Arabidopsis CDPK anti-serum and undergo in situ Ca²⁺-dependent autophosphorylation on nitrocellulose. The molecular masses of the CDPKs extracted by 8 mol/L LiCl from the cell wall fraction are different from those extracted from the cell membrane fraction, suggesting that the wall-associated CDPK is unique to the cell wall fraction. Immuno-fluorescence microscopy with isolated walls localizes CDPK to discrete punctate loci in the cell wall. Isolated Pd challenged with CDPK anti-serum show a pattern of cross-reactivity similar to the cell wall fraction. These data suggest that the cell wall-associated CDPK is a putative plasmodesmal-associated membrane protein and may be involved in regulating Pd conductivity.