Elucidation of maize endosperm starch granule channel proteins and evidence for plastoskeletal structures in maize endosperm amyloplasts

Previously, starch granules of maize (Zea mays), sorghum (Sorghum bicolor), and millet (Pennisetum americanum) were found to contain channels connecting the interior cavity with the outside surface, and the channels of maize starch channels were found to contain, presumably to be lined with, proteins. One objective of this study was to identify and characterize channel proteins of maize starch granules. A putative starch granule channel protein extract was subjected to 2D-PAGE. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was used to identify the proteins. Data analysis indicated the presence of actin-like and tubulin-like (FtsZ) proteins, ADP-glucose pyrophosphorylase large and small subunits (Shrunken2 [Sh2] and Brittle2 [Bt2], respectively), granule-bound starch synthase (GBSS), and Brittle1 (Bt1, an adenylate translocator). A combination of Western blot analysis and immunolocalization confirmed the presence of an actin-like protein within the starch granule structure. Another objective was to determine the origin of maize starch granule channels. TEM examination of maize endosperm amyloplasts that did not contain a formed starch granule revealed structures that were connected to the amyloplast membrane and extended inward to the center of the plastid.