Fig. S1. SDS/PAGE of the eluted peaks obtained in the SEC experiment shown in Fig. 1.

Fig. S2. DLS data for the His-tag-free protein.

Fig. S3. Omit map of Val60, Val61, Ala62, and Gly63, which form the open interface in domain-swapped dimers and the first inhibitory loop in monomers.

Fig. S4. Sequence alignment of the phytocystatins canecystatin-1, tarocystatin, oryzacystatin-1, and pineapple cystatin, together with human cystatin C.

Fig. S5. Comparison between canecystatin-1, stefin B, and human cystatin C.

Fig. S6. Particularities within the canecystatin-1 β-sheet.

Table S1. Angle between the helices in domain-swapped cystatin dimers.

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