Nature uses salt bridges to control the folding and stability of many proteins, including collagen, the key structural protein in mammals. Here, we present an interstrand salt bridge between (4S)-aminoproline (Amp) and aspartic acid (Asp) that directs the composition and register-specific assembly of synthetic collagen heterotrimers. This Amp-Asp salt bridge allowed for the rational design of strands that fold into A2B and ABC-type heterotrimers with only three salt bridges per triple helix. Native ESI-MS and NMR spectroscopic analyses corroborated the specific assembly of the ABC heterotrimer.