Here are five things you must know for Friday, May 31:

1. -- Stock Futures Sink as Trump Targets Mexico

U.S. stock futures fell sharply on Friday and global stocks slumped following a surprise move by Donald Trump to slap tariffs on U.S. imports from Mexico and further signals from China that its prolonged trade war with Washington continues to take a toll on the world's second-largest economy.

The president's announcement regarding Mexican tariffs, first announced through his Twitter account late Thursday, is designed to address what he has described as a wave of illegal immigration from Central America. It also suggests he is in no hurry to extend an olive branch to China, where the economy continues to show signs of contraction and export orders in the manufacturing sector have fallen for 12 consecutive months. 

Trump said that a 5% tariff will be applied to goods entering the U.S. from Mexico on June 10, and would rise to as high as 25% by Oct. 1, if the government of President Andres Manuel Lopez Obrador didn't move swiftly enough to tackle the crisis.

"If the illegal migration crisis is alleviated through effective actions taken by Mexico, to be determined in our sole discretion and judgment, the tariffs will be removed," Trump said in a statement.

Contracts tied to the Dow Jones Industrial Average tumbled 259 points, futures for the S&P 500 were down 29.65 points, and Nasdaq futures sank 93.50 points.

Government bond yields also were on the move, with investors driving benchmark 10-year U.S. Treasury bond yields to a 20-month low of 2.17% in overnight trading.

The economic calendar in the U.S. on Friday includes Personal Income and Outlays for April at 8:30 a.m. ET, Chicago PMI for May at 9:45 a.m., and Consumer Sentiment for May at 10 a.m.

Earnings reports are expected Friday from Big Lots (BIG - Get Report) and Conn's (CONN - Get Report) .

2. -- Automakers Are Hammered on Trump's Mexico Tariff Threat

Donald Trump's move to slap tariffs on goods imported from Mexico into the United States hammered the stocks of automakers Friday as investors counted the cost of price increases on one of the world's biggest car-production markets.

General Motors (GM - Get Report)  has the biggest U.S. presence in Mexico, with its three assembly plants building more than 800,000 cars last year, a 44% increase from 2010 that has drawn direct criticism from Trump as CEO Mary Berra unveiled plans to close four U.S. production facilities late last year.

GM fell 5,2% to $33.02 in premarket trading. 

Ford  (F - Get Report)  has ramped up electric vehicle production in Mexico - while unveiling sweeping global job cuts - with a move to swap a battery-electric crossover vehicle assembly plant to Cuautitlan instead of an earlier plan to build it in Flat Rock, Michigan.

Ford declined 3.9% to $9.36 in premarket trading.

European carmakers with notable Mexican exposure also were under pressure Friday.

3. -- Uber Posts a $1 Billion First-Quarter Loss 

Uber Technologies (UBER) posted a first-quarter loss of $1.01 billion, or $2.26 a share, on revenue of $3.1 billion in its first earnings report as a public company.

Shares rose 1.3% to $40.33 in premarket trading Friday. The stock began trading at $45 on May 10.

Uber had forecast a loss of $1 billion to $1.11 billion and revenue at the high end of $3.1 billion.

"Our Q1 2019 results were at or near the high end of the ranges we shared last month in our IPO prospectus," said Chief Financial Officer Nelson Chai. "Our investments remain focused on global platform expansion and long-term product and technology differentiation, but we will not hesitate to invest to defend our market position globally." 

On a conference call, Uber executives addressed, among other things, how much it must spend to maintain its dominant position and to continue growing.

"The competition is going to be more healthy, based on brand and product and technology, which is the right way to compete vs. throwing money at the problem," said Uber CEO Dara Khosrowshahi in response to an analyst question.

Uber said bookings in the quarter rose 34% from a year earlier to $14.65 billion. Monthly active users rose to 93 million globally in the first quarter, Uber said, up from 91 million at the end of the fourth quarter.

4. -- Gap Plunges on Earnings Miss, Forecast Cut

Gap Inc. (GPS - Get Report)  sunk 17% to $17.09 in premarket trading Friday after first-quarter earnings and sales missed analysts' expectations and the retailer cut its fiscal-year forecast.

The company reported first-quarter adjusted earnings of 24 cents a share on sales of $3.7 billion. Analysts had been calling for adjusted profit of 32 cents a share on sales of $3.8 billion for the quarter.

Gap said same-store sales fell 4% vs. a year-earlier gain of 1%. Same-store sales at Old Navy fell 1% and same-store sales at Gap-brand stores tumbled 10%.

"This quarter was extremely challenging," said CEO Art Peck.

The retailer said it expects fiscal-year earnings of $2.05 to $2.15 a share, down from a previous range of $2.40 to $2.55. Analysts were forecasting adjusted profit of $2.45.

Peck said Gap remains "confident in our plan to separate into two independently traded public companies in 2020." The company said in February that Old Navy would be split off as a publicly traded company.

5. -- Amazon Reportedly Interested in Buying Boost Mobile

Amazon.com  (AMZN - Get Report)  has been eyeing Boost Mobile, a prepaid mobile phone service that T-Mobile US  (TMUS - Get Report) and Sprint (S - Get Report) are looking to unload to win government approval for their proposed merger, Reuters reported.

The e-commerce and web services giant has considered the idea of buying Boost Mobile "mainly for an attached wholesale deal" that would allow it to use T-Mobile's wireless network for a half-dozen years or more and may be looking to buy any "wireless spectrum that could be divested," according to Reuters, which cited two people familiar with the plan.

An Amazon spokeswoman declined to confirm or deny the report, telling TheStreet that "we don't comment on rumors or speculation." T-Mobile and Sprint didn't respond to requests for comment late Thursday.

Reuters reported earlier this week that a group of potential buyers was preparing bids for Boost Mobile in a sale that would value the service at up to $3 billion, following the Federal Communications Commission's approval of the merger of T-Mobile and Sprint.

The deal awaits a ruling from the U.S. Justice Department.

Amazon is a holding in Jim Cramer's Action Alerts PLUS member club. Want to be alerted before Jim Cramer buys or sells AMZN? Learn more now.