IMMUNOLOGICAL DETECTION AND LOCALIZATION OF A CALSEQUESTRIN-LIKE PROTEIN IN RED BEET AND CUCUMBER CELLS

Calsequestrin is a calcium binding protein present in the sarcoplasmic reticulum (SR) of animal muscle cells and is thought to be essential for the rapid uptake and release of Ca2+, and thus for the regulation of Ca2+ -dependent cellular functions. Higher plant cells of red beet (Beta vulgaris L.) and cucumber (Cucumis sativus L.) contain a polypep tide of about M(r) 55000 that cross-reacts with a monoclonal antibody raised against calsequestrin from rabbit skeletal muscle SR, In beet t his protein changes its apparent molecular weight with pH as indicated in Western immunoblotting. Although this protein bound calcium it was not the dominant calcium-binding protein in red beet. Washing of beet root tissue leads to a slight increase of this polypeptide in microso mal fractions as indicated by immunoblotting. After immunoblotting to partially purified cell membrane fractions this polypeptide appeared t o be predominantly associated with endoplasmic reticulum-enriched frac tions. Immunogold labelling of ultrathin sections of cucumber hypocoty l using the anti-calsequestrin antibody showed that gold particles wer e very largely confined to the cytosol and often in close proximity to the ER. Clusters of up to nine gold particles were observed, often ov er small vesicular areas, as observed in some animal tissues. These re sults indicate that red beet and cucumber cells contain a protein whic h may be related to animal calsequestrin. It appears to be associated with the ER and could be involved in cellular calcium regulation.