Smithsonian Regent David Rubenstein has donated $10 million to the National Museum of African American History and Culture, one of the largest gifts made to the institution that is expected to open in September.

Rubenstein, who is co-founder and co-CEO of the Carlyle Group, has now contributed $44.7 million to the Smithsonian.

The gift — the museum's sixth donation of $10 million or more — brings its fundraising total to $252 million. The museum needs $270 million in private support to cover half of its construction costs. Congress provides the other half.

Rubenstein also will loan the museum two rare documents signed by President Abraham Lincoln: the 13th Amendment, which marked the end of slavery, and the Emancipation Proclamation. Both will be on display when the museum opens.

Lonnie Bunch, the museum's founding director, expressed his gratitude to Rubenstein, calling him "an extraordinary partner."

"This gift is another shining example of his long-standing commitment to the Smithsonian as a donor, an adviser and a fundraiser," Bunch said in a statement. "His respect for and knowledge of history, coupled with his contribution of $10 million, makes him an extraordinary partner in our commitment to explore African American history and culture in a way never attempted before — a way that makes it clear that African American history is America's history."

Rubenstein is co-chairman of the Smithsonian's $1.5 billion fundraising campaign, which includes the museum's capital needs. It has raised $1.3 billion toward its goal.

The museum will name the exhibition space at the center of the building in Rubenstein's honor. The David M. Rubenstein History Galleries will feature three exhibitions focused on slavery, segregation and the Civil Rights era.

Rubenstein said it is an honor to be among the museum's supporters. "This will be a place of learning, inspiration and healing," he said in a statement. "Designed to honor the important contributions to our country, over hundreds of years, of African Americans, it is a museum for all Americans to visit and appreciate."