Immunoglobulin binding protein

A method of isolating an immunogloblin comprising providing a solid support having bound thereto a protein and contacting a sample containing the imniunoglobulin with the support, wherein the protein bound to the support is an immunoglobulin light chain binding protein which comprises:

• (a) the amino acid sequence of SEQ ID NO:
• 1 modified by an amino acid substitution at one or more of positions 39,53 and 57 and/or by an amino acid insertion between positions 59 and 60, such that the dissociation constant (Kd) of the protein with respect to human immunoglobulin kappa chain is 400 nM or more at pH 8, or
• (b) the amino acid sequence of a corresponding immunoglobulin light chain binding domain modified by an amino acid substitution at one or more of the positions equivalent to positions 39, 53 and 57 of SEQ ID NO;
• 1 and/or by an amino acid insertion between positions equivalent to positions 59 and 60 of SEQ ID NO:
• 1, such that the dissociation constant (Kd) of the protein with respect to human immunoglobulin kappa chain is 400 nM or more at pH 8, or
• (c) the amino acid sequence of a fragment of (a) or (b) which contains at least one said substitution and/or insertion, such that the dissociation constant (Kd) of the protein with respect to human immunoglobulin kappa chain is 400 uM or more at pH8.