ALFA-tag is a sequence used in protein engineering, namely bulking up membrane proteins for cryo-EM (1). It reliably forms an alpha helix in solution with an epitope capable of binding nanobodies. Prolines break the helix at either end.
1.
Götzke H, Kilisch M, Martínez-Carranza M, Sograte-Idrissi S, Rajavel A, Schlichthaerle T, et al. The ALFA-tag is a highly versatile tool for nanobody-based bioscience applications. Nature Communications. 2019;10(1). Available from: https://doi.org/10.1038/s41467-019-12301-7