Synaptotagmin III isoform is compartmentalized in pancreatic beta-cells and has a functional role in exocytosis.
Synaptotagmin is involved in Ca2+-regulated secretion and has been suggested to serve as a general Ca2+ sensor on the membrane of secretory vesicles in neuronal cells. Insulin exocytosis from the pancreatic beta-cell is an example of a Ca2+-dependent secretory process. Previous studies of pancreatic beta-cells were unable to show presence of synaptotagmin I. We now present biochemical and immunohistochemical data showing that synaptotagmin III is present in pancreatic beta-cells as well as in the insulin-secreting cell line HIT-T15 and in rat insulinoma. By subcellular fractionation, we found synaptotagmin III in high-density fractions together with insulin and secretogranin I, indicating colocalization of synaptotagmin III and insulin in secretory granules. We could also show that blockade of synaptotagmin III by a specific antibody inhibited Ca2+-induced changes in beta-cell membrane capacitance, suggesting that synaptotagmin III is part of the functional protein complex regulating beta-cell exocytosis. The synaptotagmin III antibody did not affect the activity of the voltage-gated L-type Ca2+-channel. These findings are compatible with the view that synaptotagmin III, because of its distinct localization in the pancreatic beta-cell, functionally modulates insulin exocytosis. This indicates that synaptotagmin may have a general role in the regulation of exocytosis not only in neuronal cells but also in endocrine cells.
Brown
H
H
Department of Molecular Medicine, The Rolf Luft Centre for Diabetes Research, Karolinska Institute, Stockholm, Sweden.
Meister
B
B
Deeney
J
J
Corkey
B E
BE
Yang
S N
SN
Larsson
O
O
Rhodes
C J
CJ
Seino
S
S
Berggren
P O
PO
Fried
G
G
D000818:Animals; D000906:Antibodies / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism; D002135:Calcium-Binding Proteins; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D003594:Cytoplasmic Granules / Q000378:metabolism; D004553:Electric Conductivity; D005089:Exocytosis / Q000502:physiology*; D007150:Immunohistochemistry; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007340:Insulinoma / Q000378:metabolism; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D008297:Male; D008562:Membrane Glycoproteins / Q000276:immunology / Q000378:metabolism / Q000502:physiology*; D009419:Nerve Tissue Proteins / Q000276:immunology / Q000378:metabolism / Q000502:physiology*; D010190:Pancreatic Neoplasms / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013347:Subcellular Fractions / Q000378:metabolism; D050863:Synaptotagmin I; D050857:Synaptotagmins
D000906:Antibodies; D020746:Calcium Channels, L-Type; D002135:Calcium-Binding Proteins; D007328:Insulin; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D050863:Synaptotagmin I; C494312:Syt1 protein, rat; C494291:Syt3 protein, rat; D050857:Synaptotagmins