Alternansucrase catalyses the sequential transfer of glucose residues from sucrose onto another sucrose molecule to form a long chain polymer, known as "alternan". The alternansucrase-encoding gene from Leuconostoc citreum ABK-1 (Lcalt) was successfully cloned and expressed in Escherichia coli. Lcalt encoded LcALT of 2,057 amino acid residues; the enzyme possessed an optimum temperature and pH of 40 °C and 5.0, respectively, and its' activity was stimulated up to 2.4-fold by the presence of Mn2+. Kinetic studies of LcALT showed a high transglycosylation activity, with Km 32.2 ± 3.2 mM and kcat 290 ± 12 s-1. Alternan generated by LcALT (Lc-alternan) harbours partially alternating α-1,6 and α- 1,3 glycosidic linkages confirmed by NMR spectroscopy, methylation analysis, and partial hydrolysis of Lc-alternan products. In contrast to previously reported alternans, Lc-alternan can undergo self-assembly, forming nanoparticles with an average size of 90 nm in solution. At concentrations above 15% (w/v), Lc-alternan nanoparticles disassemble and form a high viscosity solution, while this polymer forms a transparent film once dried.