The 8-nm spaghetti: well-structured glycans coating linear tetrapeptide repeats discovered from freshwater with CryoSeek

We recently developed a research strategy, termed CryoSeek, to identify uncharacterized bio-entities from natural or endogenous resources using cryo-electron microscopy (cryo-EM). Here we report the discovery of a glycofibril whose primary molecular mass is attributed to a thick glycan shell. The 3.3-Å resolution cryo-EM reconstruction reveals that the only protein component of the glycofibril, which is approximately 8 nm in diameter, is a linear chain of tetrapeptide repeats. Each tetrapeptide repeat consists of a 3,4-dihydroxyproline (diHyp), a Ser or Thr, and two less conserved residues. Two and one glycan chains are respectively O-linked to the diHyp and Ser/Thr residues. The protein sequence pattern of this glycofibril is similar to that of our recently observed TLP-4, although the glycan chains are different. We rename the previously characterized glycofibril as TLP-4a and designate this one as TLP-4b. Our discoveries reveal the critical role of glycans in structural folding of glycoconjugates and shed light on understanding the carbon/nitrogen ratio in biospheres.