Sec12 is a guanine nucleotide-exchange factor (GEF) of the GTPbinding protein Sar1. Its GEF activity on Sar1 makes it a key element in vesicle budding from the endoplasmic reticulum to the Golgi apparatus in yeast. Sec12 is an integral membrane glycoprotein of 70 kDa. A 38.5 kDa N-cytoplasmic domain (Sec12Áp) has been expressed in Saccharomyces cerevisiae and in Escherichia coli, puri®ed to homogeneity and crystallized. Two crystal forms were obtained. Crystal form I belongs to space group P6 2 /P6 4 , with unitcell parameters a = b = 191.7, c = 53.3 A Ê , = 120 , and diffracts to 2.6 A Ê resolution. Crystal form II belongs to space group P1, with unitcell parameters a = 52.6, b = 53.0, c = 116.8 A Ê , = 98.0, = 97.4, = 93.4 , and diffracts to 2.0 A Ê resolution.