Very little is known about the in vivo regulation of the catalytic activity of the major pneumococcal autolysin (LytA), a surface-exposed enzyme that rules the self-destruction of pneumococcal cells through degradation of their peptidoglycan backbone. Two new crystal forms of the cell wall anchoring domain of LytA were obtained, and their structures were solved and refined to 2.4A and 2.8A resolution. The domain is a homodimer with a boomerang-like shape in which the tertiary structure of each monomer is comprised by six independent beta hairpins arranged in a superhelical fashion. Choline molecules at the hydrophobic interface of consecutive hairpins maintain this unique structure. The C-terminal hairpin (last 13 residues of LytA) in the solenoid is responsible for the formation of the catalytically active homodimer. Although the general fold in the structures derived from both crystal forms is essentially the same, two different conformations of the basic homodimer are observed. Biochemical approaches have demonstrated the fundamental role of the 11 C-terminal residues in the catalytic activity of LytA. The studies reported here reveal the importance of some amino acid residues at the C terminus in the determination of the relative distance of the active dimeric form of the autolysin, which appears to be essential for the catalytic activity of this enzyme.