Catalyzes the production of spermine from spermidine and decarboxylated S-adenosylmethionine (dcSAM). S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = H(+) + S-methyl-5'-thioadenosine + spermine Amine and polyamine biosynthesis; spermine biosynthesis; spermine from spermidine: step 1/1. Homodimer. Dimerization is mediated through the N-terminal domain and seems to be required for activity as deletion of the N-terminal domain causes complete loss of activity. Composed of 3 domains: the N-terminal domain has structural similarity to S-adenosylmethionine decarboxylase, the central domain is made up of four beta strands and the C-terminal domain is similar in structure to spermidine synthase. The N- and C-terminal domains are both required for activity. The disease is caused by variants affecting the gene represented in this entry. Belongs to the spermidine/spermine synthase family.