Table I.

Activities of the GST-72b′ and H68h Mutant Derivatives

SRP72 mutants	Activity (%)	Solubility	SRP68 mutants	Activity (%)
GST-k72b′	100	+++	H68h	100
ΔTPR1 (Δ11–44)	0	+++	A577–580	97 ± 9
A64L	0	+	K580A	89 ± 11
Y86A	0	+	A581–584	0
Y89A	9 ± 3	+	A581–582	85 ± 10
Δh120 (Δ113–131)	0	++++	P581A	96 ± 3
A113–116	0	++	L582A	108 ± 7
A113–114	14 ± 2	+	A583–584	56 ± 28
E113A	20 ± 6	+++	F583A	94 ± 3
L114A	8 ± 6	++	F584A	97 ± 5
A115–116	89 ± 20	++++	A585–588	0
G116A	100 ± 1	++++	A585–586	26 ± 7
A117–120	0	++++	D585A	20 ± 10
A117–118	14 ± 10	+++	L586A	60 ± 9
Q117A	82 ± 6	++++	A587–588	84 ± 17
V118A	98 ± 5	+++	A587L	84 ± 2
A119–120	0	+++	L588A	102 ± 1
L119A	18 ± 3	++	A589–592	96 ± 23
Y120A	47 ± 9	++++	A593–596	77 ± 4
A121–124	87 ± 15	++++
A125–128	16 ± 6	+
A125–126	15 ± 3	+++
Y125A	21 ± 5	+++
D126A	96 ± 6	+++
A127–128	120 ± 28	+++
A129–131	42 ± 10	+++
A130L	95 ± 12	+++
Y132A	34 ± 9	+++
ΔTPR4 (Δ132–165)	0	+++

Binding activities of the GST-72b′ mutant polypeptides were determined by measuring the protein amounts associated with Ni-NTA magnetic agarose beads which had been primed with Thx-H68e′ relative to the binding activity of GST-k72b′. The activities of mutants of H68h were determined in coexpression experiments with GST-k72b′ followed by incubation of the E. coli lysates with Ni-NTA magnetic agarose beads or GST Sepharose. The variable solubilities of the GST72b′ derivatives was assessed by centrifugation followed by SDS PAGE of polypeptides in the pellets and supernatants as indicated by ++++ (80–100%), +++ (50–80%), ++ (20–50%), and + (5–20%). Under the conditions used, H68h and all its mutant derivates exhibited favorable solubilities.