Acid prohormone sequence determines size, shape, and docking of secretory vesicles in atrial myocytes

How vesicles are born in the trans-Golgi network and reach their docking si tes at the plasma membrane is still largely unknown and is investigated in the present study on live, primary cultured atrial cardiomyocytes. Secretor y vesicles (n=422) are visualized by expressing fusion proteins of proatria l natriuretic peptide (proANP) and green fluorescent protein. Myocytes expr essing fusion proteins with intact proANP display two populations of fluore scent vesicles with apparent diameters of 120 and 175 nm, moving at a top v elocity of 0.3 mum/s. The number of docked vesicles is significantly correl ated with the number of mobile vesicles (r=0.71, P<0.0005). The deletion of the acidic N-terminal proANP[1-44] or point mutations (glu(23,24)-->gln(23 ,24)) change size and shape-but not velocity-of the vesicles, and, striking ly, abolish their docking at the plasma membrane. The shapes thus change fr om spheres to larger, irregular floppy bags or vesicle trains. Deletion of the C-terminal proANP[45-127], where the ANP and its disulfide bond reside, does not change size, shape, docking, or velocity of the mobile vesicles. The N-terminal acid calcium-binding sequence of proANP is known to cause pr otein aggregation at the high calcium concentration prevailing in the trans -Golgi network. Therefore, these results indicate that amino acid residues favoring cargo aggregation are critically important in shaping the secretor y vesicles and determining their fate-docking or not docking-at the plasma membrane. The full text of this article is available at http://www.circresa ha.org.