Thiamin-binding proteins (TBPs) occur in many kinds of plant seeds. TBPs retain thiamin in dormant seeds and supply it for germ growth during germination. It is assumed that thiamin-TBP complex is a storage form of thiamin in the seeds and thiamin is converted to thiamin pyrophosphate during germination (Mitsunaga et al. 1987). The biochemical, structural and functional properties of the TBPs are well known (Shimizu et al. 1995;Watanabe 1999;Rapala-Kozik and Kozik 1999;Adachi et al. 2000;Adamek-Swierczynska and Kozik 2002;Watanabe et al. 2003Watanabe et al. , 2004Golda et al. 2004). Many of TBPs isolated from plant seeds are globulin proteins with molecular masses over 100 kDa; however, TBPs classified into albumin proteins, with smaller molecular masses, have been found in sesame seeds (Shimizu et al. 1995) and pea seeds (AdamekSwierczynska and Kozik 2002). Pea seeds had two TBPs: globulin TBP with a molecular mass of 150-170 kDa and albumin TBP with a molecular mass of 48 kDa, and sesame seeds had three albumin TBPs, termed STBP-I, -II and -III, with molecular masses of 17-19 kDa, whereas other plant seeds had only globulin TBP with a molecular mass over 100 kDa.The three STBPs resembled each other in molecular mass, molecular structure, and binding-activity to thiamin and thiamin-related compounds. STBP-I, and -II were composed of two 8.9-kDa subunits, respectively. STBP-III was composed of two 9.3-kDa subunits (Shimizu et al. 1995). STBP-II and -III bound one molecule of thiamin per molecule, and STBP-I bound 0.5 molecule (Shimizu et al. 1995). Moreover, the amino acid sequences of small polypeptides of the three STBPs were the same . The amino acid sequences of the large polypeptides were the same, except for the C-terminus of polypeptide chains resulting in differences in molecular mass between STBP-I, -II and -III. On the other hand, cDNA encoding STBPs was isolated and characterized (Watanabe et al. 2001). These results proposed the formation of STBP-I, -II and -III from the proprotein precursor by post-translational processing. However, it remains unclear whether the three STBPs are produced from the same large proprotein precursor or encoded by a gene family (Watanabe et al. 2001). In this paper, STBPs in developing sesame seeds were analyzed by column chromatography and SDS-PAGE. Furthermore, digested Abstract The formation of thiamin-binding proteins (TBPs) from sesame (Sesamum indicum L.) seeds (STBP-I, -II and -III) in developing seeds was investigated. Elution profiles of STBPs obtained from seeds at 4 to 6 weeks after flowering from the Q-Sepharose Fast Flow column showed that only one STBP was contained in the seeds at 4 weeks, and two STBPs were contained in the seeds at 5 weeks, and all three STBPs were contained in the seeds at 6 weeks as well as in mature seeds. SDS-PAGE of those STBPs demonstrated that STBP in the seeds at 4 weeks was large STBP termed as STBP-III, the molecular mass of which was higher than STBP-I and -II. In addition, it was demonstrated that small and large STBPs were co...