TABLE 2.

Distances between residues of PLB and SERCA determined by cross-linking (X-link) versus x-ray diffraction (crystal)

Cross-linking distances between residues of PLB and SERCA were determined after co-expression of selected mutants of PLB with SERCA2a (5, 7–9) or SERCA1a (6) in microsomal membranes, or with native PLB and SERCA2a in human cardiac SR vesicles (16). Cross-linking was conducted with homo- or heterobifunctional cross-linking reagents (5, 7–9, 16) or with copper phenanthrolene (6). The distances in the PLB4/SERCA crystal structure were determined by measuring from the β-carbon of the PLB residue to the closest side chain atom of the corresponding SERCA residue (native protein). For V49G in PLB4 the distance was measured from the α-carbon of Gly⁴⁹.

PLB	SERCA	Distance (Å)		Ref.
		X-link	Crystal
N27C	Lys328	9–14	14	7
Lys27	Lys328	8	10–13a	16
N30C	Cys318	10	10	5
N30C	Lys328	14–16	17	7
L31C	T317C	10	10	8
V49C	V89C	0–5	5	6, 9
M50C	V89C	10	NDb	9
L52C	V89C	5	ND	9

^a The distance measurement between Lys²⁷ and Lys³²⁸ was estimated by computational mutation of Ala²⁷ to Lys and selecting side chain rotomers compatible with the local structure for both Lys²⁷ and Lys³²⁸, then measuring the closest approaches for the side chains.

^b ND, not determinable.